Keir Starmer has said he won’t be visiting Donald Trump next week amid rising fears about the US withdrawing support for Ukraine.
Concerns are mounting that the President-elect may cut military-aid to Kyiv when he enters the White house in January.
He has previously said he could end the war “in one day”.
Sir Keir and Emmanuel Macron pledged to put Ukraine in the ”strongest possible position” as they held talks in Paris today.
Their meeting follows suggestions that Britain and France could seek to persuade Joe Biden to grant Ukraine permission to use Storm Shadow missiles to strike into Russia before he leaves the White House.
Mr Biden is said to have blocked the UK from authorising Ukraine to use the missiles over fears of retaliatory attacks on Western military bases.
World leaders around the globe are anticipating how drastically Trump intends to change US policy on Ukraine following his presidential victory last week.
Questions have been raised following Trump’s presidential election victory about what his second term could mean for US support for Ukraine and Nato.
Over the weekend the Washington Post reported that the President-elect has already spoken to Vladimir Putin – urging the Russian leader not to further escalate the war.
The Kremlin denied a phone call took place, claiming the article “does not correspond at all to reality”.
A number of world leaders have said they will visit Mr Trump at his Mar-a-Lago home on their way to the G20 summit in Rio de Janeiro next week.
But Sir Keir said he currently has no plans to visit the president-elect on his way to Brazil.
“No we’re going straight there as far as I know,” he told journalists en-route from Paris to Azerbaijan, where he is due to attend the COP 29 climate summit.
“To be perfectly honest, next week is a long way off just at the moment, but as far as I know we’re going straight there.
“I obviously spoke to him as you know on the phone on Wednesday evening – a very constructive, very good conversation.
“And obviously I had the opportunity a few weeks ago to speak to him for a couple of hours in New York where we discussed a wider range of issues and we’ll continue to do so.”
Sir Keir met Mr Macron ahead of Armistice Day commemorations marking the end of the First World War, becoming the first British prime minister to attend the events in the French Capital since Winston Churchill in 1944.
Macron personally invited Sir Keir to mark the 120th anniversary of the Entente Cordiale between France and the UK, which paved the way for greater diplomatic cooperation between the two countries.
A statement from the Elysee Palace said the two leaders had reaffirmed their commitment to “support Ukraine unwaveringly”.
Downing Street said the pair discussed how to put Ukraine in “the strongest possible position going into the winter”.
But Sir Keir refused to confirm whether the two leaders discussed the use of Storm Shadows in Russian territory.
“I’ve always said that we support Ukraine for as long as it takes, and we’ve stood by that,” he said.
“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”.
Downing Street insists “no war was ever won by a single weapon” when asked about the potential use of the missiles.
A Number 10 spokeswoman said: “Our position on Storm Shadow hasn’t changed.
“We’ve always said that when we discuss our support for Ukraine, we do so in terms of broader strategy to ensure that Ukraine is in the strongest possible position going forwards, particularly in the winter, and we’ve also been clear that no war was ever won by a single weapon.”
During the Armistice Day commemorations, Sir Keir also met French Prime Minister Michel Barnier before observing a minute’s silence.
The PM and Macron laid a wreath at the statue of Churchill in Paris and travelled in an open roof car to review troops around the Arc de Triomphe.
During their meeting the two leaders also committed to “cooperation on migration in the Channel, particularly in the face of human trafficking networks,” a statement said.
The pair discussed European security and “the prospects for the relationship” between the EU and the UK, alongside discussions the humanitarian situation in Gaza.
Earlier, Defence Secretary John Healey earlier downplayed concerns that the incoming Trump administration could imperil Ukraine, insisting he expects US support for Kyiv to continue “for as along as it takes to prevail” over Moscow.
During his campaign, the president-elect has insisted he would find a solution to end the war “within a day” without explaining how he would do so, which some have interpreted to mean a peace on terms favourable to Moscow.
He has also expressed scepticism towards Nato, accusing European countries of free-riding on America.
But facing broadcasters on Monday morning, Mr Healey said he believed Washington would remain “steadfast” in its backing and would not turn away from the military bloc.
“As far as President Trump goes, he recognises that countries get security through strength, just as alliances like Nato do, and I expect the US to remain alongside allies like the UK, standing with Ukraine for as long as it takes to prevail over Putin’s invasion,” he told Sky News.
“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.”
