Keir Starmer and French President Emmanuel Macron said they want to put Ukraine in the "strongest possible position going into winter" amid uncertainty over US backing.
The pair met in Paris hours after Donald Trump's son claimed Ukrainian President Volodymyr Zelensky is "38 days from losing (his) allowance".
Donald Trump Jr, whose dad will return to the White House in January, made the remark in an Instagram video. The President-elect has claimed he could end the war within 24 hours - sparking concerns he could push for huge concessions to warmonger Vladimir Putin.
Mr Starmer met the French leader ahead of commemorations marking the end of the First World War. He became the first British PM to attend the Paris event since Winston Churchill in 1944.
No10 yesterday(MON) refused to say whether Mr Starmer believes Ukraine will be safe under a second Trump presidency. But it said that it expects Washington to "remain alongside allies like the UK". Downing Street said any peace talks should be "led by Ukraine."
Defence Secretary John Healey said he expects US support for Kyiv to continue "for as long as it takes to prevail" over Moscow.
Speaking to reporters on the way to the COP29 summit in Baku, Azerbaijan, Mr Starmer said: “I've always said that we support Ukraine for as long as it takes, and we’re stood by that.
"I've also said that on a number of occasions, we need to put Ukraine in the best possible position. It has been my constant refrain for four months now, since I've been Prime Minister, in all the discussions that I've had.
"That is the heart of the discussions that we've been having. We did, of course, discuss Ukraine this morning, as you would expect, and for reasons that you'll already understand, I'm not going to go into the operational matters.
"But full support for Ukraine and making sure Ukraine is in the best possible position, have been the sort of driving principles of the arguments that I've been putting forward”.
There has been speculation that Mr Trump could push for a peace deal that would see the current front lines frozen. Ukraine would also be barred from joining NATO for 20 years under the deal, according to insiders.
But Mr Zelensky said it would be "suicide" for Europe to make concessions to Putin. There have also been concerns about Mr Trump's commitment to NATO.
Mr Healey tried to bat away these concerns, stating: "I don't expect the US to turn away from NATO. They recognise the importance of the alliance. They recognise the importance of avoiding further conflict in Europe."