Keir Starmer is expected to join European allies for an emergency summit in Paris over the Ukraine war.
The prime minister said it was a “once in a generation moment for our national security” and that the continent must take a larger role in Nato.
The meeting is likely to be held on Monday and will address fears the US is looking to cut Europe out of its peace talks with Russia.
Allies will likely to discuss how the continent should address Ukraine’s future membership of Nato and how to offer Kyiv further security guarantees.
It comes after Donald Trump’s special envoy to Ukraine hinted that US and Russia will hold negotiations on how to resolve the war and will consult, but not directly involve, Europe.
Special envoy Keith Kellogg claimed previous negotiations fell apart because too many parties were involved.
He may have been alluding to the 2015 Minsk Agreements, negotiated by Europe, which failed to secure a ceasefire with Russia in the east of Ukraine.
Kellogg told the Munich conference that negotiations “may be like chalk on the blackboard, it may grate a little bit, but I am telling you something that is really quite honest”.
Senior American figures and Russian negotiators are expected to meet in Saudi Arabia in the coming days.
The US claims Ukraine is invited, but Ukrainian president Volodymyr Zelenskyy said Kyiv is yet to receive an invitation.
According to the Guardian, Downing Street confirmed on Saturday that it heard of the proposed meeting, that Starmer would attend, and that he would take messages from Europe to Washington when he visits later this month.
Some reports suggest he will then meet with European leaders and Zelenskyy upon his return.
The prime minister said: “This is a once-in-a-generation moment for our national security where we engage with the reality of the world today and the threat we face from Russia.
“It’s clear Europe must take a greater role in Nato as we work with the United States to secure Ukraine’s future and face down the threat we face from Russia.
“The UK will work to ensure we keep the US and Europe together. We cannot allow any divisions in the alliance to distract from the external enemies we face.”
The meeting will test Starmer’s mission to stay perfectly balanced between the US and Europe in a post-Brexit world.
While France is yet to announce the talks, Polish foreign minister Radoslaw Sikorski said at the Munich security conference that French president Emmanuel Macron had called for the summit.
Sikorski said: “President Trump has a method of operating, which the Russians call reconnaissance through battle.
“You push and you see what happens, and then you change your position, legitimate tactics. And we need to respond.”
Meanwhile, Zelenskyy has called for a creation of an “army of Europe” after US vice-president JD Vance attacked the continent on Friday, saying its greatest threat came “from within”.
The Ukrainian president claimed the old relationship between Europe and America was “ending” and the continent “needs to adjust to that”.
The Ukrainian leader also said Kyiv would “never accept deals made behind our backs”.
Foreign secretary David Lammy said the UK and the EU have to spend more on defence as Europe faces an “existential question” even if a peace is negotiated in Ukraine.
Trump has been pushing for Nato countries to spend 5% of their GDP on defence – a huge increase compared to the 2% they are expected to spent now.
Lammy also admitted at the Munich conference “we all know we have to go upward”.