Europe must "ramp up" pressure on Russia "to bring an end to this barbaric war", a Downing Street spokesperson said after Sir Keir Starmer chaired a meeting with the Coalition of the Willing.
Earlier, the prime minister met with Ukrainian President Volodymyr Zelenskyy, French President Emmanuel Macron and German Chancellor Friedrich Merz at Downing Street for crisis talks on the future of Ukraine.
After the Downing Street meeting, a spokesperson said Sir Keir "convened a call with other European allies alongside President Zelenskyy".
"The leaders all agreed that now is a critical moment and that we must continue to ramp up support to Ukraine and economic pressure on Putin to bring an end to this barbaric war."
Ukraine latest: 'Critical stage' in push for peace
During the earlier meeting, the Downing Street spokesperson said that the leaders "discussed the importance of the US-led peace talks for European security and supported the progress made".
All four "underscored the need for a just and lasting peace in Ukraine, which includes robust security guarantees," the spokesperson added.
"The leaders agreed that, while diplomatic efforts continue, Europe must stand with Ukraine, strengthening its ability to defend against relentless attacks that have left thousands without heat or light," they said.
"They also discussed positive progress made to use immobilised Russian sovereign assets to support Ukraine's reconstruction."
After arriving in the UK, Mr Zelenskyy said in a post on Telegram that "we are joining our efforts to end this war with a decent peace for Ukraine" and that "security must be guaranteed".
He later said: "We can't manage without Americans, we can't manage without Europe and that is why we need to make some important decisions."
The discussions come as US President Donald Trump accused Mr Zelenskyy of having failed to read the latest peace plan.
He said he was "a little disappointed" in the Ukrainian leader, while insisting Russia's Vladimir Putin was "fine with it".
Mr Trump's eldest son had earlier told Sky's lead world news presenter Yalda Hakim in an interview that his father may walk away from the Ukraine peace process.
When asked directly if he believed the US president would walk away from Ukraine, he answered: "I think he may, what's good about my father and what's unique about my father is you don't know what he's going to do. He's unpredictable."
Sir Keir had earlier insisted any ceasefire must be "just" and "lasting", adding: "We are at a critical stage in the push for peace.
"The principles remain the same. We stand with Ukraine, and if there is to be a ceasefire, it needs to be a just and lasting ceasefire."
Mr Macron had insisted Ukraine's allies have "a lot of cards" - but the main issue was to "finalise these peace negotiations" and find the "best possible conditions for Ukraine, for the Europeans, and for collective security".
Moments after arriving at Downing Street, Mr Merz said he was "sceptical about some of the details in documents coming from the US" but that was one of the reasons for Monday's talks.
The four European leaders emerged from Downing Street at around 2.20pm for a group photo - but declined to answer any questions from the media.
Sir Keir and Mr Zelenskyy then went back inside the prime minister's residence for further talks while Mr Macron and Mr Merz departed.
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The London meeting comes after Mr Zelenskyy's officials concluded three days of talks with US diplomats.
Over the weekend, Mr Zelenskyy said he had discussed "next steps" with Mr Trump's advisers and was "determined to keep working in good faith".
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