Sir Keir Starmer confirmed he is prepared to put a peacekeeping force into Ukraine if there is a deal to end the war with Russia.
Sir Keir made the commitment while acknowledging this could put British troops "in harm's way" if Russian President Vladimir Putin attacks again.
The Prime Minister is joining European leaders for emergency talks in Paris on Monday as they scramble for a strategy in response to US President Donald Trump's push for a deal with Russia and fears the US will reduce its defence commitments in the continent.
See also: Starmer warned of ‘considerable cost’ of sending peacekeeping troops to Ukraine
Sir Keir said the crisis was a "once in a generation moment" and an "existential" question for Europe.
Writing in The Daily Telegraph, the Prime Minister said the UK was "ready to play a leading role" in Ukraine's defence and security, including the commitment of £3 billion a year until 2030.
But he said that along with military aid to Ukraine "it also means being ready and willing to contribute to security guarantees to Ukraine by putting our own troops on the ground if necessary".
"I do not say that lightly. I feel very deeply the responsibility that comes with potentially putting British servicemen and women in harm's way," Sir Keir said.
"But any role in helping to guarantee Ukraine's security is helping to guarantee the security of our continent, and the security of this country.
"The end of this war, when it comes, cannot merely become a temporary pause before Putin attacks again."
Trump has pledged talks with Vladimir Putin to end the war in Ukraine, with US officials on Sunday heading to a meeting with Russian officials to discuss the war.
Kyiv has said it is not attending the talks as it was not invited - despite the US administration insisting it had been.
Sir Keir will later this month visit Trump in Washington, where he is expected to set out European concerns about the White House’s plan.
A further meeting of European leaders, including Ukrainian President Volodymyr Zelensky, is likely when Sir Keir returns from his US trip.
The meeting on Monday will see Mr Macron and Sir Keir joined by leaders from Germany, Italy, Poland, Spain, the Netherlands and Denmark along with the presidents of the European Council and European Commission and the secretary general of Nato.
Business Secretary Jonathan Reynolds earlier said the UK was well placed to try to keep the Western allies together - but admitted it would not be easy.
He told BBC's Sunday With Laura Kuenssberg: "There is definitely, from the US administration, a view towards Europe and there's a slightly different view towards ourselves in the UK.
He added: “I'm not saying it's going to be easy, there's a very assertive agenda from the US."
Spanish Foreign Minister Jose Manuel Albares said European leaders will discuss how to prevent a peace negotiation on Ukraine ending up rewarding Russian aggression.
"A war of aggression cannot be rewarded, we cannot encourage others to launch wars of aggression," he said in an interview with radio station Onda Cero.
"Today I'm convinced Putin will keep attacking and bombing Ukraine. So I do not see peace on the horizon at the moment," he added.
Mr Reynolds also said the need for increased defence spending is widely recognised, including by Chancellor Rachel Reeves.
The Sunday Times reported that Sir Keir held one-to-one meetings on Friday with the Chief of the Defence Staff Admiral Sir Tony Radakin amid calls for higher defence spending.
Labour has pledged to increase spending from its current 2.3% of gross domestic product to 2.5%, although no timetable has been set out for that increase.
The Chancellor will set out the multi-year plan for public spending on June 11, which is expected to set out how the UK will increase the share spent on defence to 2.5%.
Shadow defence secretary James Cartlidge said the Munich Security Conference underlined that the US was more focused on China and Europe would have to do "more of that heavy lifting" in respect of Russia.
He told GB News: "What we've had in Munich is basically a wake-up call, which is that we do need to do more as European Nato members, we do need to spend more on defence, and that's true in the United Kingdom."
The US’ upcoming talks in Saudi Arabia will be among the first high-level in-person discussions between Russian and US officials in years.
The US Middle East envoy, Steve Witkoff, who on Sunday confirmed the talks, denied ostracising Ukraine from discussions on its future, telling Fox News: “I don't think this is about excluding anybody. In fact, it's about including everybody."
