Sir Keir Starmer has clashed with Donald Trump on whether Ukraine should aspire to join Nato ahead of today's crunch White House summit.
The US President had suggested that Ukrainian leader Volodymyr Zelensky's could help bring about an end to the conflict "almost immediately" if he drops his plan to join the bloc.
However, the Prime Minister's position is that Kyiv has the sovereign right to join both Nato and the European Union.
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“Russia cannot have a veto against Ukraine’s pathway to the EU or Nato," the Prime Minister's official spokesman said.
Sir Keir has so far established a healthy relationship with Mr Trump.
During a recent visit to Scotland, the US President described the Prime Minister as a "friend" and steered clear of criticising the Labour leader.
However, Mr Trump's attack against Sir Sadiq Khan put Sir Keir in an uncomfortable position, forcing the Prime Minister to defend the London Mayor while not antagonising the US President.
After Mr Trump labelled the London Mayor a "nasty man", Sir Keir said: "He's a friend of mine, actually."
Sir Keir Starmer and Donald Trump
|PA
However, the US President will not find many friends in the UK when it comes to Ukraine's relationship with Nato.
Ex-Prime Minister Boris Johnson, a one-time despatch box sparring partner for Sir Keir in the House of Commons, last year wrote that it is time to let the ex-Soviet state join the military alliance.
Reform UK leader Nigel Farage was perhaps seen as the only mainstream political figure in Britain to reflect Mr Trump's concerns about Ukraine's potential Nato membership.
The Brexit supremo had previously accused Nato and the European Union of "provoking" Russia's invasion.
Volodymyr Zelensky
| REUTERSHowever, Mr Farage appeared to change tack earlier this year, admitting it is "probably essential" for Ukraine to join the military bloc.
The Clacton MP said: "If we can get Putin into a place, and Zelensky into a place where we have a reasonable negotiation, I would have thought looking ahead, it’s probably essential that Ukraine joins Nato."
Meanwhile, Mr Trump remains hellbent on stopping Ukraine joining Nato, ruling out any expansion to accommodate the ex-Soviet state during crunch peace negotiations.
In a post on Truth Social, Mr Trump said: "President Zelensky of Ukraine can end the war with Russia almost immediately, if he wants to, or he can continue to fight.
Vladimir Putin | REUTERS"Remember how it started. Not getting back Obama given Crimea (12 years ago, without a shot being fired!), and NO GOING INTO NATO BY UKRAINE. Some things never change."
Sir Keir will join Mr Zelensky and Mr Trump in the White House at around 5.15pm today.
The Prime Minister is expected to pose for a "family photo" alongside European leaders later this evening.
In a video released while Sir Keir was flying to the US, the Prime Minister said: "This was in Ukraine has been going on a really long time now, three plus years."
Prime Minister Sir Keir Starmer (left) and President of Ukraine Volodymyr Zelensky speak to the media at the UK Ambassador's Residence in Paris | PA"It's hugely impacted the Ukrainians, who have suffered hugely, but it's also impacted Europe, it's impacted every single family and community in the United Kingdom.
"Everybody wants it to end, not least for the Ukrainians, but we've got to get the right.
"We've got to make sure there's peace, but it's lasting peace, and that it is fair and that it is just.
"That's why I am travelling to Washington, with other European leaders, to discuss this face-to-face with President Trump and President Zelensky.
"It is in everyone's interest, it's in the UK's interest, to get this right."