Should that happen, General Sir Gwyn Jenkins said it would be the first time since the end of the Second World War that Britain's warships and submarines were not the dominant force in their most vital sea lanes alongside their allies.
"We are holding on, but not by much," he told a conference in London on Monday.
"There is no room for complacency. Our would-be opponents are investing billions. We have to step up, or we will lose that advantage."
As a senior, serving military officer speaking publicly, he did not make any direct criticism of the speed of plans by Sir Keir Starmer's government to increase defence spending.
But Sky News has reported that he and his fellow chiefs held a "very difficult meeting" last month over how to fund plans to rebuild the armed forces amid fears of further cuts.
Defence sources said there was growing concern at the very top of the armed forces about a gap between the promises being made by the prime minister to fix the UK's hollowed-out defences and the reality of the size of the defence budget, which is currently not seen as growing fast enough.
That means either billions of additional pounds must be found more quickly, or ambitions to modernise and transform the armed forces might need to be curbed, despite warnings of mounting threats from Russia and China, and pressure from Donald Trump on allies to spend more on their own defences.
A Sky News and Tortoise podcast series called The Wargame tracks the hollowing out of the UK's military since the end of the Cold War and the risk that has created.
👉Search for The Wargame on your podcast app👈
General Jenkins, the first Royal Marine to serve as First Sea Lord, used a speech at the Sea Power Conference to say that Russia is still investing billions in its naval capabilities - in particular the Northern Fleet that operates in the Atlantic - even as it wages war against Ukraine.
There has been a 30% increase in Russian incursions in the North Atlantic in the past two years, he said.
That included the Yantar spy ship, which last month was spotted off the coast of Scotland and even shone a laser at the pilots of a Royal Air Force reconnaissance plane that was tracking the vessel.
Yet General Jenkins said what Russia is doing beneath the surface of the waves, where the UK and its allies store vital communications cables as well as critical oil and gas pipelines, was even more concerning.
"I can also tell you today that the advantage that we have enjoyed in the Atlantic since the end of the Second World War is at risk," he said.
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Navy facing huge challenges
It is a particularly tough time for the navy, which has more ships and submarines alongside and unable to operate than at sea or at least ready to sail.
The service is also suffering from a shortage of sailors and in particular submariners, which again is impacting the availability of the fleet.
The crisis follows decades of funding cuts since the end of the Cold War, compounded by a litany of botched procurement programmes that has all too often seen vessels coming into service years late, at an inflated price and in too few numbers.
Vision of 'hybrid navy'
Despite the sombre tone, the First Sea Lord set out how he wants to transform his service and make it ready to fight a war - though not until 2029, a timeline that could be too slow if some predictions about the threat posed by Russia to NATO are correct.
His vision - working with industry and other allies - is about developing a blend of manned ships and submarines as well as unmanned ones - a "hybrid navy".
He is also stripping back what he called the navy's own bureaucracies to enable the service to move much faster - crucially at the pace of the threat and the pace of rapid and growing technological change.
"We will face headwinds, we will face rough seas, but together, we can solve these problems if we have the appetite, if we have the determination, and if we have the mindset."
Quite the opposite.
Ukraine's military chief says Vladimir Putin is instead using the US president's focus on peace negotiations as "cover" while Russian soldiers attempt to seize more land.
That means much greater pressure on the Ukrainian frontline, even as Russian and American, or American and Ukrainian, or Ukrainian and European, leaders shake hands and smile for cameras before retreating behind closed doors in Moscow, Alaska, and London.
Putin's not counting on peace
The lack of any indicators that the Kremlin is looking to slow its military machine down also makes the risk of war spreading beyond Ukraine's borders increasingly likely.
It takes a huge amount of effort, time, and money to put a country on a war footing as Putin has done, partially mobilising his population, allocating huge portions of government spending to the military and realigning Russia's vast industrial base to produce weapons and ammunition.
But when the fighting stops, it requires almost as much focus and energy to switch a society back to a peace time rhythm.
Deliberately choosing not to dial defence down once the battles cease means a nation will continue to grow its armed forces and weapons stockpiles - a sure sign that it has no intention of being peaceful and is merely having a pause before going on the attack again.
The absence of any preparations by Moscow to slow the tempo of its military operations in Ukraine - where it has more than 710,000 troops deployed along a 780-mile frontline - is perhaps an indicator that Putin is anticipating more not less war.
How could the war end?
What happens next in Europe will depend on the content of any peace deal on Ukraine.
An all-out Russian defeat is all but impossible to conceive without a significant change of heart by the Trump White House and a massive increase in weapons and support.
The next best result for Ukraine would be a settlement that seeks to strike a fair balance between the warring sides and their conflicting objectives.
This could be done by pausing the fighting along the current line of contact before substantive peace talks then take place, with Ukraine's sovereignty supported by solid security guarantees from Europe and the US.
But such a move would require Europe's NATO allies, led by the UK, France and Germany, genuinely to switch their respective militaries and populations back to a wartime footing, with a credible readiness to go to war should Moscow attempt to test their support of Ukraine.
Will Starmer level with the public?
That does not just mean increased spending on defence at a much faster rate - in the UK at least - than is currently planned. It is also about the mindset of a country and its willingness to take some pain.
France is already openly saying that parents may have to lose their children in a war with Russia, while Germany is requiring all 18-year-old men to undergo medical checks for possible national service.
No such tough but frank conversation is being attempted by Sir Keir Starmer with the British public.
The furthest his military chief has gone is to say "warfighting readiness" is his top priority.
But that is meaningless jargon for most of the public. Being ready for war is about so much more than what the professional armed forces can do.
Armies fight battles. Countries fight wars.
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Worst case scenario?
The other alternative when it comes to Ukraine is a scenario that sees a sidelined Europe unable to influence the outcome of the negotiations and Kyiv forced to agree to terms that favour Moscow.
This would include the surrender of land in the Donbas that is still under Ukrainian control.
Such a deal - even if tolerated by Ukraine, which is unimaginable without serious unrest - would likely only mean a temporary halt in hostilities until Putin or whoever succeeds him decides to try again to take the rest of Ukraine, or maybe even test NATO's borders by moving against the Baltic States.
With Trump's new national security strategy making clear the US would only intervene to defend Europe if such a move is in America's interests, it is no longer certain that the guarantees contained in NATO's founding Article 5 principle - that an attack on one member state is an attack on all - can be relied upon.
To have a sense of how a war with Russia might play out without the US on NATO's side, Sky News and Tortoise ran a wargame that simulates a Russian attack on the UK.
In the scenario, Washington does not come to Britain's defences, which leaves the British side with very few options to respond short of a nuclear strike.
Train services already in public ownership will begin to adopt a Union flag-inspired design from this spring, Heidi Alexander has confirmed.
It's part of the government's efforts to nationalise most passenger rail services in the UK and run them under the new Great British Railways (GBR) brand - with its logo to be unveiled later today.
Ms Alexander will be speaking more about GBR - which is due to be formally established in 2027 - on Mornings With Ridge And Frost from around 7.10am.
Haven't I seen this somewhere before?
The branding features the familiar double arrow symbol used by British Rail when the country's trains were last state-owned.
A spokesperson for the Department for Transport (DfT) said the "iconic" symbol has been incorporated into the new GBR logo to reflect "Britain's proud railway heritage".
Ms Alexander has insisted it "isn't just a paint job", saying: "It represents a new railway, casting off the frustrations of the past and focused entirely on delivering a proper public service for passengers."
Special one-day public exhibition launched
People in the capital will be able to see the new train livery for themselves today, with a special one-day exhibit being held at London Bridge station, where a GBR-branded Hornby model train will be on display.
The government has also partnered with a gaming company to create mock-ups of the new design, and those at the station will be able to see a digital demonstration of the new artwork in Train Sim World 6.
The new designs will also be beamed on to digital display boards over the coming days at Manchester Piccadilly, Birmingham New Street, Glasgow Central, and Leeds City.
It comes as MPs prepare to debate the government's Railways Bill in the Commons.
The government has taken control of seven major operators so far, but has pledged to return all passenger services to public ownership as contracts with existing operators expire or are broken through a failure to deliver.
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The government has said its Railways Bill will bring 17 different organisations together and will also lead to greater accountability, better services and an easier way to buy tickets.
Among the measures is the creation of a GBR app, where passengers will be able to check train times and buy tickets without booking fees, while those with disabilities will be able to also request assistance.
The draft law would also beef up accountability by creating a strengthened Passenger Watchdog, while GBR would operate both services and maintain the railways themselves.
Ministers have pointed to improvements to existing services since they have been nationalised, such as South Western Railway boosting capacity by almost 10% by quadrupling its number of Arterio trains in service.
A new East Coast Main Line timetable will also come into effect this month, which the government says will lead to 10,000 extra LNER services every year, or roughly 60,000 extra seats a week.
The former deputy prime minister delighted Labour backbenchers with a powerful Commons speech defending her workers' rights legislation on Monday evening.
With the House of Lords locked in a battle of parliamentary "ping pong" with MPs, she told ministers: "Now is not the time to blink or buckle."
Her very public intervention came amid claims that her next move has the Labour Party on tenterhooks and that she's the favourite to succeed Sir Keir if she wants the job.
And her speech, delivered from notes and clearly meticulously prepared, appeared to send a message to Labour MPs: I'm here to make a comeback.
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The government's flagship Employment Rights Bill was championed by Ms Rayner when she was deputy PM, in the face of bitter opposition from the Conservatives.
In a bid to end the deadlock with the Lords, ministers have backed down on unfair dismissal protection from day one, proposing a compromise of six months.
Backing the compromise, brokered with the TUC, Ms Rayner said: "I know ministers had faced difficult decisions and difficult discussions with the employers and worker representatives.
"But I strongly believe that the work that has been done has been necessary, and we should be able to move forward now."
'No excuses'
Attacking the upper chamber for delaying the legislation, she said: "There is now no more time to waste.
"Vested interests worked with the Tories and the Lib Dems and, cheered on by Reform and backed by the Greens, to resist the manifesto on which we were elected.
"And now there can be no excuses. We have a mandate for a new deal for working people, and we must, and we will deliver it.
And she concluded: "It has been a battle to pass this bill, but progress is always a struggle that we fought for. Its passage will be a historic achievement for this Labour government.
"It will benefit working people now and into the future. Now is not the time to blink or buckle. Let's not waste a minute more. It's time to deliver."
Could she replace her old boss?
It was the sort of fighting talk and defiance of the government's opponents that will have cheered up Labour MPs and boosted her hopes of a comeback and even a leadership bid.
Speculation over Sir Keir's future grows more frenzied by the day, with claims that even some of his own supporters have begun the hunt for his successor.
The thinktank that ran his leadership campaign in 2020, Labour Together, is reported to be canvassing party members on candidates to replace him.
There was even a claim last week that allies of Wes Streeting were sounding out Labour MPs about a pact with Ms Rayner and a joint ticket for the leadership.
The health secretary dismissed that claim as a "silly season story", while a Rayner ally said: "There's no vacancy and there's no pact". They added that she will not "be played like a pawn".
Mr Streeting did, however, start speculation himself when he said in his Labour conference speech: "We want her back. We need her back."
Fuelling more speculation, Sir Keir went further than he had previously on Sunday, when he was asked in an Observer interview if he missed her and replied: "Yes, of course I do. I was really sad that we lost her."
And asked if she would return to the cabinet, the prime minister said: "Yes. She's hugely talented."
Sir Keir also described Ms Rayner, who left school at 16 without any qualifications, as "the best social mobility story this country has ever seen".
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But a swift return to the cabinet would be hugely controversial, because the PM's ethic adviser, Sir Laurie Magner, ruled that she breached the ministerial code by underpaying stamp duty when she bought a flat.
But she has been linked to speculation about possible efforts to remove Sir Keir if - as predicted - Labour performs badly in the Scottish, Welsh and local elections next May.
Her supporters also claim she will eventually be cleared by HMRC over her stamp duty breach, clearing the way for her to come back.
And her latest speech - combative, defiant and yet loyal - will have boosted her hopes, and reminded Labour MPs what they've missed since she quit in September.
An investigation into NHS maternity services is under way after a series of shocking scandals.
The National Maternity and Neonatal Investigation (NMNI) is being led by Baroness Amos, who said "nothing prepared her" for the amount of "unacceptable care" families currently receive.
A report has been released documenting her initial reflections and impressions after meeting families and visiting hospitals.
She will investigate 12 NHS trusts in total, including Oxford University Hospitals (OUH), which runs the world-renowned John Radcliffe Hospital.
'I was left in my own blood'
Rebecca Matthews formed a campaign for families failed by OUH after her own traumatic births.
Asked to discuss the care she received, she said she "could only describe it as callous".
"There wasn't any kindness there. I was left in my own blood," she added.
Ms Matthews recently took part in evidence-gathering sessions held by Baroness Amos.
But when she read her interim report, she said it was "disappointing", as it appeared to be "a bullet point list of failings that actually we've seen time and time again in independent reviews".
"The reflections don't mention accountability at all," she said.
'Why are we struggling to provide?'
Based on her initial inquiries, Baroness Amos found common themes, including women not being listened to and being "disregarded" when they raised concerns.
Many weren't given the right information to make informed choices about their care.
She was told of discrimination against women of colour, working-class mothers, or parents who were younger.
A "staggering" 748 recommendations have been made about NHS maternity services in recent years, Baroness Amos revealed - and she does "not understand why change has been so slow".
She asked: "Why are we in England still struggling to provide safe, reliable maternity and neonatal care everywhere in the country?"
The most recent health watchdog findings paint a depressing picture of maternity services.
Almost two-thirds of acute hospital maternity services were judged either inadequate or required improvement for safety.
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This investigation is long overdue and isn't due to report back fully until the spring.
But some campaigners are already worried it won't bring meaningful change to maternity services.
Ms Matthews said it "seems as though it's heading the same way that other reviews have gone in the past, leading to some recommendations but no teeth".
"We need some mechanisms that are going to hold people and systems to account," she said.
'More to do'
OUH chief nurse Yvonne Christley said in a statement that "feedback received from patients using our maternity service over the last year is positive overall".
"However, we know we have more to do to improve our maternity services," she added.
"Our present focus is on listening to the experiences of women and families, which is helping us to identify opportunities for improvement."
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