Baroness Heather Hallett, the chair of the inquiry, described the response to the pandemic as "too little, too late".
Tens of thousands of lives could have been saved during the first wave of COVID-19 had a mandatory lockdown been introduced a week earlier, the inquiry also found.
Noting how a "lack of urgency" made a mandatory lockdown "inevitable", the report references modelling data to claim there could have been 23,000 fewer deaths during the first wave in England had it been introduced a week earlier.
The UK government first introduced advisory restrictions on 16 March 2020, including self-isolation, household quarantine and social distancing.
Had these measures been introduced sooner, the report states, the mandatory lockdown which followed from 23 March might not have been necessary at all.
COVID-19 first emerged in the Chinese city of Wuhan at the end of 2019, and as it developed into a worldwide pandemic, the UK went in and out of unprecedented lockdown measures for two years starting from March 2020.
Lady Hallett admitted in her summary that politicians in the government and devolved administrations were forced to make decisions where "there was often no right answer or good outcome".
"Nonetheless," she said, "I can summarise my findings of the response as 'too little, too late'".
'Toxic culture' at the heart of UK government
The report said there was "a toxic and chaotic culture" at the heart of the UK government during the pandemic.
The inquiry heard evidence about the "destabilising behaviour of a number of individuals" - including former No 10 adviser Dominic Cummings.
It said that by failing to tackle this chaotic culture - "and, at times, actively encouraging it" - former PM Boris Johnson "reinforced a culture in which the loudest voices prevailed and the views of other colleagues, particularly women, often went ignored, to the detriment of good decision-making".
'Misleading assurances'
The inquiry found all four governments in England, Scotland, Wales and Northern Ireland failed to understand the urgency of response the pandemic demanded in the early part of 2020.
The report reads: "This was compounded, in part, by misleading assurances from the Department of Health and Social Care and the widely held view that the UK was well prepared for a pandemic."
The report notes how the UK government took a "high risk" when it significantly eased restrictions in England in July 2020 - "despite scientific advisers' concerns about the public health risks of doing so".
Lady Hallett has made 19 key recommendations which, if followed, she believes will better protect the UK in any future pandemic and improve decision-making in a crisis.
Repeated failings 'inexcusable'
In a statement following the publication of Thursday's report, Lady Hallett said there was a "serious failure" by all four governments to appreciate the level of "risk and calamity" facing the UK.
She said: "The tempo of the response should have been increased. It was not. February 2020 was a lost month."
Read more:
A timeline of the UK's response to the pandemic
Lady Hallett said the inquiry does not advocate for national lockdowns, which she said should have been avoided if at all possible.
She said: "But to avoid them, governments must take timely and decisive action to control a spreading virus. The four governments of the UK did not."
Lady Hallett said none of the governments were adequately prepared for the challenges and risks that a lockdown presented, and that many of the same failings were repeated later in 2020, which she said was "inexcusable".
She added: "Each government had ample warning that the prevalence of the virus was increasing and would continue to do so into the winter months. Yet again, there was a failure to take timely and effective action."
They tried to undermine inquiry chair Lady Hallet's attempt to understand what went wrong and how we might do better, and portray it as a lame exercise that would achieve very little.
Well, we now know that Boris Johnson's "toxic and chaotic" government could well have prevented at least 23,000 deaths had they acted sooner and with greater urgency.
Follow latest: All four UK governments 'failed to appreciate' scale of COVID pandemic threat
The response was "too little, too late". And nobody in power truly understood the scale of the emerging threat or the urgency of the response it required.
The grieving families who lost loved ones in the pandemic want answers. They want names. And they want accountability.
But that is beyond the remit of this inquiry.
Read more:
'Toxic and chaotic culture' at centre of UK government during the pandemic
A timeline of the UK's response to the pandemic
Everything you need to know about the COVID inquiry
The publication of the report into Module 2 of the inquiry will bring them no comfort, it may even cause them more distress.
But it will bring them closer to understanding why the UK's response to this unprecedented health crisis was so poor.
We can easily identify the "advisers and ministers whose alleged rule breaking caused huge distress and undermined public confidence".
And we know who was in charge of the Department of Health and Social Care as it misled the public by giving the impression that the UK was well prepared for the pandemic when it clearly was not.
The 28-point proposal was cooked up between Trump negotiator Steve Witkoff and Kremlin official Kirill Dmitriev without European and Ukrainian involvement.
It effectively dresses up Russian demands as a peace proposal. Demands first made by Russia at the high watermark of its invasion in 2022, before defeats forced it to retreat from much of Ukraine.
Ukraine war latest: Kyiv receives US peace plan
Its proposals are non-starters for Ukrainians.
It would hand over the rest of Donbas, territory they have spent almost four years and lost tens of thousands of men defending.
Analysts estimate at the current rate of advance, it would take Russia four more years to take the land it is proposing simply to give them instead.
It proposes more than halving the size of the Ukrainian military and depriving them of some of their most effective long-range weapons.
And it would bar any foreign forces acting as peacekeepers in Ukraine after any peace deal is done.
The plan comes at an excruciating time for the Ukrainians.
They are being pounded with devastating drone attacks, killing dozens in the last few nights alone.
They are on the verge of losing a key stronghold city, Pokrovsk.
And Volodymyr Zelenskyy is embroiled in the gravest political crisis since the war began, with key officials facing damaging corruption allegations.
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The suspicion is Mr Witkoff and Mr Dmitriev conspired together to choose this moment to put even more pressure on the Ukrainian president.
Perversely, though, it may help him.
There has been universal condemnation and outrage in Kyiv at the Witkoff-Dmitriev plan. Rivals have little choice but to rally around the wartime Ukrainian leader as he faces such unreasonable demands.
The genesis of this plan is unclear.
Was it born from Donald Trump's overinflated belief in his peacemaking abilities? His overrated Gaza ceasefire plan attracted lavish praise from world leaders, but now seems mired in deepening difficulty.
The fear is Mr Trump's team are finding ways to allow him to walk away from this conflict altogether, blaming Ukrainian intransigence for the failure of his diplomacy.
Mr Trump has already ended financial support for Ukraine, acting as an arms dealer instead, selling weapons to Europe to pass on to the invaded democracy.
If he were to take away military intelligence support too, Ukraine would be blind to the kind of attacks that in recent days have killed scores of civilians.
Europe and Ukraine cannot reject the plan entirely and risk alienating Mr Trump.
They will play for time and hope against all the evidence he can still be persuaded to desert the Kremlin and put pressure on Vladimir Putin to end the war, rather than force Ukraine to surrender instead.
But unless John Healey backs his rhetoric up with a far more urgent push to rearm - and to rebuild wider national resilience - he risks his words ringing as hollow as his military.
The defence secretary on Wednesday repeated government plans to increase defence spending and work with NATO allies to bolster European security.
Instead of focusing purely on the threat, he also stressed how plans to buy weapons and build arms factories will create jobs and economic growth.
In a sign of the government's priorities, job creation is typically the top line of any Ministry of Defence press release about its latest investment in missiles, drones and warships rather than why the equipment is vital to defend the nation.
I doubt expanding employment opportunities was the motivating factor in the 1930s when the UK converted car factories into Spitfire production lines to prepare for war with Nazi Germany.
Yet communicating to the public what war readiness really means must surely be just as important today.
Mr Healey also chose this moment of national peril to attempt to score political points by criticising the previous Conservative government for hollowing out the armed forces - when the military was left in a similarly underfunded state during the last Labour government.
A report by a group of MPs, released on the same day as Mr Healey rattled his sabre at Russia, underlined the scale of the challenge the UK faces.
It accused the government of lacking a national plan to defend itself from attack.
The Defence Select Committee also warned that Mr Healey, Sir Keir Starmer and the rest of the cabinet are moving at a "glacial" pace to fix the problem and are failing to launch a "national conversation on defence and security" - something the prime minister had promised last year.
The report backed up the findings of a wargame podcast by Sky News and Tortoise that simulated what might happen if Russia launched waves of missile strikes against the UK.
The series showed how successive defence cuts since the end of the Cold War means the army, navy and air force are woefully equipped to defend the home front.
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But credible national defences also require the wider country to be prepared for war.
A set of plans setting out what must happen in the transition from peace to war was quietly shelved at the start of this century, so there no longer exists a rehearsed and resourced system to ensure local authorities, businesses and the wider population know what to do.
Mr Healey revealed that the Russian spy ship had directed a laser light presumably to dazzle pilots of a Royal Air Force reconnaissance aircraft that was tracking it.
"That Russian action is deeply dangerous," he said.
"So, my message to Russia and to Putin, is this: We see you. We know what you are doing. And if Yantar travels south this week, we are ready."
He did not spell out what this might mean but it could include attempts to block the Russian vessel's passage, or even fire warning shots to force it to retreat.
However, any direct engagement could trigger a retaliation from Moscow.
For now, the Russian ship - fitted with spying equipment to monitor critical national infrastructure such as communications cables on the seabed - has moved away from the UK coast. It was at its closest between 5 and 11 November.
The military is still tracking its movements closely in case the ship returns.
Asked what he thought of the US president's decision, the prime minister simply said Mr Trump had "set out his position".
The PM added he thought it was "really important to be [at the G20] to talk to other partners and allies so we can get on with the discussions around global issues that have to be addressed, and do have an impact back at home, but also to take the opportunity face to face to further the deals that I want to do for our country".
Sir Keir has faced heavy criticism at home for the amount of time he has spent overseas and focusing on international affairs. His trip to South Africa to attend the G20 summit is the 45th country the prime minister has visited since taking office.
Speaking to journalists on the flight over to Johannesburg, Sir Keir defended his decision to fly out days before a difficult budget, saying that the international issues being discussed in South Africa have an impact at home, while the G20 nations are important to Britain's economy.
"The G20 are the 20 strongest economies in the world, they are very important to the UK," he said.
"In the last three years, the jobs that have been generated in the UK from countries in the G20 is 200,000 and that focus in the budget will be very much the economy and the cost of living. I will focus on the deals we can do, the business we can do with our partner countries and make sure that the work we do internationally is impacting directly at home in the positive sense, that if you want to deal with the cost of living and make people better off, good, secure jobs with investment from G20 partners and allies is really important."
As part of these efforts, the government will announce £400m worth of export deals with South Africa during the summit.
This summit is the first one in the G20's 26-year history that a US president has not attended, with one diplomatic source acknowledging this was raising serious concerns. They said: "Trump also made the argument that the G7 should be the G8 [at the last meeting in Canada in June] and now he's not even going to the G20, so his lack of attendance is, of course an issue."
Mr Trump has also ordered US officials not to travel to South Africa for the annual meeting, although the country's president, Cyril Ramaphosa, said on Thursday evening that this might change, with discussions now under way with the US.
While Mr Trump is not attending, Sir Keir will leave the G20 summit early, coming back to the UK on Sunday to prepare for a tax-raising, and possibly manifesto-breaking budget on Wednesday.
The chancellor raised £40bn in taxes in the last budget, insisting that this was a "once in a parliament" tax raid. A year on, Rachel Reeves now has to raise billions more as she looks to fill a black hole as much as £30bn in the public finances, driven in part by a downgrade in productivity, which has lowered growth forecasts, and also her reversal on spending cuts - the winter fuel allowance and disability benefits - that has left her with around £7bn to find.
The government has U-turned on its plan to raise income tax but is expected to extend a freeze on tax thresholds by two years from 2028. The measure will raise about £10bn in additional tax as workers find themselves dragged into higher tax bands and has led to accusations that Labour has broken its manifesto pledge not to raise taxes on working people.
The prime minister, asked whether everyone should expect tax rises in the budget on Wednesday, refused to answer directly. Instead, he said it would be "a Labour budget with Labour values" and based on "fairness".
He added: "It will have absolutely in mind protecting our public services, particularly the NHS, cutting our debt, and dealing with the cost of living, bearing down on the cost of living. So, they'll be the principles that will run through the budget.
"Now, of course, the right decisions have to be taken. And we have to see this in the context of 16, 17 years now where we've had the crash in '08, followed by austerity, followed by a not very good Brexit deal, followed by Covid, followed by Ukraine, and that's why we have to take the decision to get this back on track.
"I'm optimistic about the future, I do think if we get this right, our country has a great future. They'll be the principles behind the budget."
While the prime minister is focusing on trade at the G20 summit, Ukraine will also be on the agenda amid reports the Trump administration and Russian officials have drawn up a new peace plan to end the war there.
It would require major concessions from Kyiv, including giving up territory not currently occupied by Russia to Moscow and halving the size of the Ukrainian army. The deal has reportedly been drawn up by Mr Trump's special envoy Steve Witkoff, who met the current secretary of the national security and defence council of Ukraine and former defence minister, Rustem Umerov, in Miami.
Asked about the plan, Sir Keir said he wanted a "just and lasting peace", adding: "The future of Ukraine must be determined by Ukraine, and we must never lose sight of that".
I'm told by one diplomatic source that the Europeans have yet to see this plan, while there are questions as to how advanced in the US administration these proposals are and whether they have the support of Secretary of State Marco Rubio.
European diplomats are stressing that any peace talks have to involve both Ukraine and European input if it is to have any hope of working. Kaja Kallas, the EU's foreign policy chief, said on the eve of the G20 summit they are yet to see any concessions on the Russian side.
"We welcome all meaningful efforts to end this war, but like we have said before, it has to be just and lasting," she said. "That also means that the Ukrainians, but also the Europeans, agree to this."
European leaders are discussing how to best equip Kyiv for another winter of war. Talks are expected to continue this weekend over the plan to use Russia's frozen assets to generate a €140bn loan for Ukraine.
The plan is currently stalled over Belgium's concerns of legal risk in releasing funds from the Brussels-based depository Euroclear, where most of the Russian assets are held.
Earlier on Thursday, Sky News revealed Sir Keir is preparing for a likely visit to China in the new year. The trip is likely to be controversial given the UK's fractious relationship with China, made worse by recent allegations of spying in parliament.
Sir Keir said any visit was not confirmed "yet" and insisted the government would "always robustly protect our interests".