Multiple European nations will form a "coalition of the willing" to present a Ukraine peace plan to the United States.
The move is a vital step to make Washington offer security and deter Russia, according to Kyiv.
The announcement by British Prime Minister Keir Starmer followed Ukraine's Volodymyr Zelenskyy being publicly berated by US President Donald Trump and Vice President JD Vance in the Oval Office.
Mr Zelenskyy cut short his trip to Washington and travelled to London just two days later.
Sir Keir said Britain, Ukraine, France and other countries would form the "coalition".
"This is not a moment for more talk," he said.
"It's time to act. Time to step up and lead and to untie around a new plan for a just and enduring peace."
'Coalition of the willing' to keep Kyiv at involved in peace talks
More than two dozen European leaders attended the London meeting on Sunday local time.
However it was not clear how many of them had agreed to contribute to the effort.
Sir Keir said "a number of countries" had indicated they wanted to be part of the peace plan being developed, but did not name the nations.
"I'll leave them to make their own statements about exactly how they want to make that contribution," he said.
He added leaders had agreed to ensure Kyiv would be at the table for any peace talks and those willing would "intensify planning now with real urgency".
Sir Keir said the leaders also agreed to work to ensure Kyiv was at the table of any peace talks and boost the country's own defence capabilities.
The term — "coalition of the willing" has previously been used by President Bill Clinton in relation to North Korea and by George W Bush to introduce the 2002 National Security Strategy.
It was also used to refer to the US military command during the 2003 invasion of Iraq and much of the Iraq war.
Stepping up defence investment 'of utmost importance'
While no leader gave details of the plan, French President Emmanuel Macron said before the summit the plan would involve a one-month ceasefire.
The ceasefire proposed by Mr Macron would apply to air and sea attacks, but not ground combat.
He also told French newspaper Le Figaro European troops would be deployed if a more substantial peace agreement was reached.
It was not clear whether other nations had agreed to these terms.
Mr Zelenskyy said Ukraine would not cede any territory to Russia as part of a peace agreement.
Other leaders said they must increase defence spending, including European Commission President Ursula von der Leyen.
"After a long time of underinvestment, it is now of utmost importance to step up the defence investment for a long period of time," she said.
She added Europe needed to turn Ukraine into a "steel porcupine" that was "indigestible for potential invaders".
NATO Secretary General Mark Rutte said some European leaders had privately set out new plans on defence spending, but he declined to give details.
Trump says war must end 'quickly' or not at all
Meanwhile Mr Trump has said talks to end Russia's invasion of Ukraine were "very well advanced".
No formal talks involving both Kyiv and Moscow have taken place despite US officials meeting with Russian delegates last month.
It has now been three years since Russia invaded Ukraine on February 24, 2022.
The invasion, the deadliest conflict in Europe since World War II, marked a major escalation of the Russo-Ukrainian War which began in 2014.
Mr Trump made the remarks during Sir Keir visit to the White house last week — a visit made to urge the US to continue supporting Ukraine.
Mr Trump said he believed Russian President Vladimir Putin would "keep his word" and not push to continue the war following any ceasefire.
"I think we've made a lot of progress, and I think it's moving along pretty rapidly," he said.
"If it doesn't happen quickly, it may not happen at all."
The Trump administration has also continued to criticise Mr Zelenskyy, with White House national security adviser Mike Waltz telling CNN the US needed a Ukrainian leader willing to secure lasting peace.
Zelenskyy promises diplomacy with America
It was not clear when the plan would be finalised or when it would be presented to the US.
Mr Zelenskyy said he still believed the relationship between the US and Ukraine was salvageable, promising more diplomatic efforts during his nightly address on Monday.
"There will be diplomacy for the sake of peace," he said.
"And for the sake of us all being together. Ukraine, all of Europe, and definitely, definitely America."
"Of course we are aware of the importance of America, and we are grateful for all the support we have received from the United States of America.
"There has not been a day when we have not felt this gratitude."
[zelenskyy] Mr Trump took to his Truth Social platform to break his silence on the situation in Europe, urging his followers to "spend less time worrying about Putin".
Instead he warned against "migrant rape gangs, drug lords, murderers, and people from mental institutions" entering the US, a topic he frequently touched on during his 2024 presidential campaign.
He was scheduled to speak during a joint session of Congress on Tuesday local time.
The address, just six weeks after his inauguration, should set out Mr Trump's vision for the United States and the world.
ABC/Reuters/AFP
