The agreement states that a "multinational force for Ukraine" would be deployed to the eastern European country to deter further attacks from Russia and bolster security guarantees.
This would also allow the Ukrainian armed forces to rebuild strength, Sir Keir Starmer has announced.
It follows a meeting of the so-called Coalition of the Willing in Paris on Tuesday afternoon, led by French President Emmanuel Macron and the British prime minister.
Jared Kushner, the US president's son-in-law, and the US special envoy, Steve Witkoff, also attended the talks, along with a number of other European leaders, including Ukrainian President Volodymyr Zelenskyy.
Sir Keir has previously stated that the UK might deploy troops to Ukraine following a peace deal, but this agreement now creates the legal framework for allowing British and French troops to operate there.
The declaration was formally unveiled at a news conference by Mr Macron, Ukrainian President Volodymyr Zelenskyy and Sir Keir.
Addressing the media, Sir Keir said: "We've had a very constructive meeting.
"The purpose of the Coalition of the Willing is to help to deliver a peace that can last and to work with the US to guarantee Ukrainian security for the long term.
"This work is more advanced than ever."
He then laid out the terms of the declaration.
The PM said the UK would participate in US-led "monitoring and verification of any ceasefire", would provide arms to Ukraine in the long-term, and would seek binding commitments to support Ukraine in case of a future armed attack by Russia.
As well as ensuring that soldiers would be deployed on Ukrainian soil, the agreement also covers protection for Ukraine's skies and sea, Sir Keir added.
The declaration also states that the two countries would establish "military hubs" in Ukraine, so that military equipment could be deployed for defensive needs.
Sir Keir added: "This is all about building the practical foundations on which peace would rest. But we can only get to a peace deal if Putin is ready to make compromises.
"We have to be frank - for all Russia's words, Putin is not showing that he is ready for peace."
He concluded that the UK and its allies will continue to turn up the pressure on Russia until "they come to the table in good faith".
Speaking at the news conference, Mr Macron said the summit had seen European nations discuss "strong security guarantees" with the US representatives, while Mr Zelenskyy called the joint declaration "very concrete".
The Ukrainian president added: "This illustrates the willingness of the coalition and the European countries to work for peace.
"We want to be ready so that when diplomacy reaches peace, we can deploy the forces of the Coalition of the Willing."
Ukraine already has enough details and knows which countries will get involved in the agreement, including what each of them will contribute separately, Mr Zelenskyy said.
But he admitted that some issues "remain open" with the revised 20-point peace plan, with the biggest problem being "the issue of territory".
Speaking on behalf of the US, Mr Witkoff said he would meet with the Ukrainian delegation again on Tuesday evening, but that they are "largely finished with security protocols".
He added: "We are here to mediate and help in the peace process, and we're prepared to do anything necessary to get to that place. President Trump strongly, fervently believes that this killing has to stop, that the carnage has to stop."
Donald Trump's son-in law, Mr Kushner, later said the agreement is "a very, very big milestone", in answer to a question from a journalist.
The French meteorological service, Meteo France, has named it as Goretti, and the south of the UK is expected to be most heavily affected on Thursday and Friday.
Sky News meteorologist Dr Christopher England said that the "exact storm track is a little uncertain" but added that the "most likely scenario is heavy rain spreading from the southwest on Thursday turning to snow from around Wales across to East Anglia".
He said that "significant falls" were likely, along with "severe gales around southern coasts".
As it happened: 'Multi-hazard' event on way this week
The UK Health Security Agency (UKHSA) has extended its amber cold health alerts across England until midday on Sunday.
The warnings came after what the Met Office has reported as "the coldest night of the winter so far," and as a major incident was declared in Aberdeenshire.
Aberdeenshire council said that expects a "prolonged period of significant impacts" over the weather, warning of:
• Travel delays on roads are likely, stranding some vehicles and passengers;
• Some delays and cancellations to rail and air travel are likely;
• There is a good chance that some rural communities could become cut off;
• Injuries from slips and falls are likely on icy surfaces;
• Untreated pavements and cycle paths likely to be impassable;
• There is a good chance that power cuts may occur, with the potential to affect other services, such as mobile phone coverage.
New weather warnings issued by Met Office
The office has issued fresh weather warnings for Wednesday, Thursday and Friday.
Three yellow warnings in effect on Wednesday. One, for ice, covers Northern Ireland. A second warning for snow and ice is also in effect across northern mainland Scotland, part of the Outer Hebrides, and the Orkney Islands.
And a third - for ice - covers much of the rest of UK except southwest England and the west coasts of Wales and Scotland.
Check the latest weather forecast where you are
Thursday sees two new yellow warnings: One for wind in the southwest of England, and one for snow in much of Wales, the Midlands, Bath and Bristol, and Oxford.
The snow warning lasts until midday on Friday, and warns that in some areas "5-10 cm of snow may settle with the potential for up to 20 cm in some locations, especially over higher ground".
Two amber warnings for snow are in effect in Scotland throughout Tuesday, as hundreds of schools in Shetland, Orkney and Aberdeenshire remained shut.
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Meanwhile, Monday night was "the coldest night of the winter so far," according to the Met Office, after a low temperature of -12.5C (9.5F) was recorded in Marham, Norfolk.
Those waking up in London at 5am faced temperatures down to -5C (23F), with overnight lows of -11.2C (11.84F) in Dalwhinnie, in the Scottish Highlands.
In a post on social media, the Met Office said: "Last night was the coldest night of the winter so far, making it a very cold and frosty start for most of us this morning."
National Rail said that train services in northern Scotland will be disrupted until the end of the day on Tuesday.
South Yorkshire Police (SYP) informed the family of Trevor Wynn, 17, that he had passed away, along with a girl of the same age, following a serious collision on Todwick Road in Rotherham, on 13 December.
At the time, it had been thought that Joshua Johnson, 18, was under sedation in hospital after being injured.
But on Sunday, concerns were raised about the identity of those killed in the incident, prompting SYP to carry out further checks, including forensic testing, after "information came to light".
Those tests confirmed that Mr Wynn was the badly injured youth in hospital receiving treatment, and Mr Johnson was the person who died.
Families of both teenagers will be offered the support of specialist services, police said.
Assistant Chief Constable Colin McFarlane said it came as "a huge shock to everyone, and we recognise the additional trauma this may cause.
"We are supporting Trevor and all the families through this and have engaged specialist agencies to help provide that support.
"I have also offered to meet with both sets of parents, as I am sure they will have many questions, most of which we are not able to answer yet, but we are absolutely committed to understanding how this happened so it cannot happen again.
"We have taken the decision to refer ourselves to the Independent Office for Police Conduct (IOPC).
"We will co-operate fully with any subsequent investigation and will be led by the IOPC on the next steps to determine how this happened, and how we can ensure this never happens again in the future."
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An 18-year-old was arrested on suspicion of causing death by dangerous driving and a 19-year-old was held on suspicion of perverting the course of justice.
Both have been bailed pending further inquiries.
In its statement, SYP said: "All of those involved have asked that we thank the public for their support but ask that they are now left in peace to process the future they now face."
Amorim had managed Manchester United since November 2024 and was contracted to the club until the end of next season, in summer of 2027.
Ending his contract 18 months early means Manchester United could face a £12m payout, Sky Sports News reports.
The payout would come during a financially difficult time for the club, which have made a loss six years in a row.
Since 2014, Manchester United have seen ten new managers - nearly one each year. Over the same period, they have paid out almost £86m in "costs associated with loss of office" to former team managers, coaches, other senior staff, and board members.
The Sky News Data x Forensics team analysed the club's financial statements filed with the US Securities and Exchange Commission to calculate the cost of these payouts.
Now, Amorim's estimated payout could push the total to nearly £98m.
High manager turnover
The high cost of Manchester United's payouts can largely be explained by high manager turnover over the past few years, said Kieran Maguire, associate professor in football finance at the University of Liverpool.
"There's been a lot of churn at Manchester United," said Mr Maguire. "Each time you have a reset, there is a cost involved as far as the club is concerned."
Since Sir Alex's retirement in 2013 after 27 years, the club has had six permanent managers and four interim managers.
"You could argue, it's a reflection of a lack of success because the clubs don't dismiss managers if they're successful," said Mr Maguire.
Although new managers are often signed on for several years, many do not last the full length of their contract.
In the post-Ferguson era, Manchester United managers in total have left the club with over 147 months - around 4,500 days - still left on their contracts.
Most notably, David Moyes left less than one year into his six-year contract. The payout was at just £5m, according to reports at the time, far from the £36m which he would have earned over the full contract.
The terms of these contracts vary, but many have break clauses which allow for early dismissals if certain conditions, such as qualifying for the Champions League, aren't met, explained Mr Maguire.
The Big Six
Despite Manchester United's high turnover rate, Chelsea and Tottenham have had the most managers among the traditional Big Six English Premier League clubs since 2010.
Newly appointed Liam Rosenior will become Chelsea's sixteenth manager in as many years, following Enzo Maresca's departure last week. Since 2010, the team has rarely seen a manager make it past the two-year mark.
Chelsea has spent £193m in payouts towards former managers, senior staff, executives, and board members since 2010, according to Mr Maguire's calculations.
Tottenham's current manager Thomas Frank has been at the helm since the end of last season. The club has had twelve managers - counting separately Ryan Mason's two interim stints - since 2010.
On the other hand, Liverpool, Arsenal, and Man City have ushered fewer managers through their doors.
Though Liverpool saw rapid management turnover in the early 2010s, the club have seen more stability in recent years with only two managers in the past decade.
Meanwhile, Arsenal have remained in mostly steady hands, with Mikel Arteta entering his sixth year as manager, following a short leadership stint from Unai Emery and several decades under Arsène Wenger.
Man Utd's financial woes
Manchester United's managerial woes are matched by financial ones, its filings show.
The club has incurred losses six years in a row, with £39.7m lost in their most recent financial year.
The club may also face additional, less obvious costs from replacing Amorim beyond the payments owed to him and departing staff from his core team.
The £98m figure excludes the costs of recruiting new head coaches. For instance, Sky Sports News reported last year that releasing Ruben Amorim from his previous contract with Sporting Lisbon cost Manchester United around £9.2m.
Once brought in, new managers often - although not always - choose to reshape the club's squad, said Mr Maguire.
"If a new manager wants to impose a particular style of football, then that becomes quite an expensive exercise because the manager will have identified playing talent that they want to recruit," he said. "Therefore, it can often coincide with an increase in transfer spend."
However, the costs of dismissing Amorim are likely ones the club are willing to swallow on the hopes they can improve their standings. Qualification for the Champions League, Mr Maguire said, is worth between a minimum of £40m and more than £150m if they secure the top place.
Manchester United have failed to break into the top five in the Premier League since the 2022/23 season.
"Clubs such as Chelsea and Manchester United want to be in the Champions League and are therefore willing to incur a financial cost in sacking a manager if they believe that will allow them to get over," said Mr Maquire.
"With Manchester United sitting sixth in the Premier League, the club's leadership has reluctantly made the decision that it is the right time to make a change," read the club's statement announcing Amorim's departure.
"This will give the team the best opportunity of the highest possible Premier League finish."
Avon and Somerset Police said they had received a number of reported sightings of Matthew Armstrong and Daniel Washbourne, who absconded from HMP Leyhill.
Armstrong was convicted of murdering someone during a robbery in Warwickshire in 2009.
Washbourne has previous convictions for offences of false imprisonment and violence against a person, police said.
On Saturday, two men matching their descriptions were pictured in Ashley Road, Bristol at around 10.35am. They are seen walking towards the Stokes Croft area.
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Then, on Sunday, at around 9.35am-9.45am, a man is seen walking along Filton Road and then Gloucester Road North in Filton, stopping at one stage to look at bus timetables.
The man is wearing a burgundy coat matching the one worn by Armstrong when he left HMP Leyhill.
Police believe he continues to walk along the same road and is seen in the Patchway area at about 11.05am-11.15am.
Armstrong is 35 years old and described as a white male, about 5ft 9ins, with ginger hair and has links to Warwickshire.
Washbourne, 40, is described as white, male, about 5ft 6ins, slim, with brown hair, clean shaven and has links to Herefordshire.
Three men were initially reported to have absconded from the South Gloucestershire prison between 5-8pm on Thursday 1 January.
A 39-year-old man was arrested by Avon and Somerset Police in Bristol on the afternoon of Saturday 3 January.
He has since been charged with escaping lawful custody and appeared in court yesterday. He is due to attend Bristol Crown Court next month.