Sir Keir Starmer has hailed “real progress” as European leaders, including Volodymyr Zelensky, held talks with Donald Trump aimed at bringing an end to the war in Ukraine.
Describing an end to the war as crucial for “the security of the UK”, the prime minister said the meeting marked a “historic step” towards peace in the war-torn region.
Sir Keir joined Mr Zelensky, French president Emmanuel Macron and Nato secretary general Mark Rutte for the crunch talks on Monday.
US president Donald Trump said he spoke directly with Vladimir Putin to begin planning a meeting between the Russian leader and Mr Zelensky, which will then be followed by a three-way meeting.
Mr Trump said Moscow would "accept" multinational efforts to guarantee Ukraine's security.
And, following the talks, Sir Keir said the talks were “good and constructive”. “There was a real sense of unity between the European leaders that were there, and president Trump and president Zelensky,” the PM said.
He touted "two material outcomes" from the talks, firstly that the coalition of the willing "will now work with the US" on security guarantees.
"That's really important for security in Ukraine, for security in Europe, and for security in the UK," Sir Keir said.
"The other material outcome was the agreement that there will now be a bilateral agreement between president Putin and president Zelensky, that was after a phone call between president Trump and president Putin during the course of this afternoon, followed by a trilateral which will then add president Trump.
"That is a recognition of the principle that on some of these issues, whether it's territory or the exchange of prisoners, or the very serious issue of the return of children, that is something where Ukraine must be at the table.
"These were the two outcomes that were the most important coming out of today. They're positive outcomes, there was a real sense of unity. We've made real progress today."
Mr Trump also described the talks as “very good” on his Truth Social platform, adding: “During the meeting we discussed security guarantees for Ukraine, which guarantees would be provided by the various European countries, with a coordination with the United States of America.
"Everyone is happy about the possibility of PEACE for Russia/Ukraine.
"At the conclusion of the meetings, I called president Putin and began arrangements for a meeting, at a location to be determined, between president Putin and president Zelensky.
"After the meeting takes place, we will have a trilat which would be the two presidents plus myself."
He later added: “We also need to discuss the possible exchanges of territory taken into consideration the current line of contact.”
Mr Trump's envoy, Steve Witkoff, had suggested measures similar to Nato's article five mutual defence provision - that an attack on one member is an attack on the entire bloc - could be offered by the US without Kyiv joining the alliance.
"We were able to win the following concession: that the United States could offer article five-like protection, which is one of the real reasons why Ukraine wants to be in Nato," Mr Witkoff told CNN over the weekend, as he spoke about the Alaska summit.
Future three-way talks "have a good chance" of stopping the conflict, the US president claimed.
But the president appeared to share conflicting views on whether a ceasefire was necessary to stop the war.
"I don't think you need a ceasefire," he had originally said, before later explaining: "All of us would obviously prefer an immediate ceasefire while we work on a lasting peace.”
During the discussions, Sir Keir welcomed plans for a security guarantee, after Mr Trump introduced him at the negotiating table as a "friend" on Monday.
He said: "Your indication of security guarantees, of some sort of Article Five-style guarantees, fits with what we've been doing with the coalition of the willing which we started some months ago, bringing countries together and showing that we were prepared to step up to the plate when it came to security.
"With you coming alongside, the US alongside, what we've already developed, I think we could take a really important step forward today - a historic step, actually, could come out of this meeting in terms of security for Ukraine and security in Europe."
Sir Keir also described potential future trilateral talks as appearing to be a "sensible next step" and continued: "So, thank you for being prepared to take that forward, because I think if we can ensure that that is the progress out of this meeting - both security guarantees and some sort of progress on (a) trilateral meeting of some sort to bring some of the difficult issues to a head - then I think today will be seen as a very important day in recent years."
The prime minister was understood to have disrupted his holiday plans over the weekend to join calls before he headed to Washington, as reported in The Times.
Following the talks, Mr Trump said he also discussed "the massive worldwide problem of missing children" with European Commission president Ursula von der Leyen.
Responding on X, Mr Zelensky said: "This issue lies at the heart of the war's humanitarian tragedy - our children, broken families, the pain of separation. At least 20,000 children were taken.
"We are working tirelessly to bring every child home. The same applies for our prisoners of war and civilians held in Russia for years, some since 2014, in very bad conditions. Thousands of people still need to be freed - and this is a part of making peace.
"We will work to negotiate all-for-all prisoners of war exchange, and we are grateful to have strong friends who help."
Mr Zelensky, whom Mr Trump greeted at the door of the West Wing with a handshake earlier in the evening, wore a black shirt with buttons and a black blazer to the meeting at the White House.
His attire had appeared to become a point of irritation for Mr Trump during a previous meeting in February.
Early in the meeting, the Ukrainian described the talks as "really good", saying they had been "the best" so far.
Mr Zelensky said: "We are very happy with the president that all the leaders are here and security in Ukraine depends on the United States and on you and on those leaders who are with us in our hearts."
