Sir Keir Starmer will join European leaders in presenting a united front with Volodymyr Zelensky at his crunch meeting at the White House with Donald Trump.
The Prime Minister and six other political leaders will travel to Washington DC on Monday, with the aim of protecting Ukraine from having to submit to Russian land grabs as a price for peace.
Those joining Sir Keir include France’s Emmanuel Macron, Germany’s Friedrich Merz, Italy’s Giorgia Meloni and Alexander Stubb, president of Finland.
Nato chief Mark Rutte and European Commission president Ursula von der Leyen are also attending.
Mr Zelensky is expecting to face calls from the US president to concede to full Russian control of Donetsk and Luhansk, two mineral-rich regions of Ukraine that are mostly occupied by Vladimir Putin’s forces.
In exchange for these demands, the Russian president would reportedly withdraw his forces from other areas of Ukraine and accept a Nato-like guarantee that Ukraine would be protected from further incursion.
The European leaders have said it is up to Ukraine to decide how it wishes to end the war, and hailed Mr Zelensky’s commitment to a peace that is both “just and lasting”.
Mr Trump has appeared to drop his calls for a ceasefire after a summit in Alaska with his Russian counterpart on Friday.
Mr Putin has long refused to agree to a ceasefire as a precondition for talks to end the war, prompting fears that Russia could continue gaining ground in Ukraine as negotiations take place.
The US president has instead said he wants to focus a long-term peace deal, though his secretary of state Marco Rubio has signalled a deal is “still a long ways off”.
There will be “additional consequences” for Russia if it does not agree to a peace deal, Mr Rubio added, though he suggested fresh financial sanctions would be unlikely to force Mr Putin to the negotiating table.
Ms von der Leyen suggested at a press conference on Sunday that both a ceasefire and a peace deal would have the same impact: to “stop the killing”.
Appearing alongside her, Ukraine’s Mr Zelensky appeared to agree, though he also signalled his preference for a ceasefire.
“It’s impossible to do this under the pressure of weapons. So it’s necessary to cease fire and work quickly on a final deal,” he said.
European leaders are also keen to hear from Mr Trump after he signalled he would provide a security guarantee to the coalition of the willing.
The coalition, which is aimed at deterring future Russian aggression once peace is agreed, has argued it needs an American backstop, likely in the form of air support, to succeed.
Over the weekend, Sir Keir was among the leaders who welcomed suggestions from Mr Trump that he was open to providing a guarantee, but details of what support would be provided were scant.
Following a meeting of the coalition on Sunday afternoon, a Downing Street spokesman said Sir Keir praised Mr Zelensky’s desire for a “just and lasting peace” in Ukraine.
Leaders of the coalition “reaffirmed their continued support to Ukraine” at the meeting chaired by the PM and Mr Macron, No 10 added.
The French president, meanwhile, said the European delegation will ask Mr Trump to back its plans to bolster Ukraine’s armed forces.
Ahead of their Oval Office encounter, the allies are likely to be mindful of the previous occasion Mr Zelensky visited Mr Trump in the White House.
February’s public spat, which saw Vice President JD Vance accuse Mr Zelensky of not being thankful enough to the US, resulted in American aid to Ukraine being temporarily halted.
