Sir Keir Starmer called on the US to provide a “backstop” if a peacekeeping force with British troops is deployed in Ukraine.

The Prime Minister made the demand following a meeting with European leaders in Paris for emergency talks on the war.

Sir Keir said: “Europe must play its role, and I’m prepared to consider committing British forces on the ground alongside others if there is a lasting peace agreement.

“But there must be a US backstop, because a US security guarantee is the only way to effectively deter Russia from attacking Ukraine again.”

The PM described Ukraine as an “existential question” for Europe as a whole.

He added that it was time for the continent to “take responsibility” for its security.

But the Prime Minister’s peace plan was facing a humiliating snub by Germany amid European splits over sending troops to Ukraine.

France was expected to back the Prime Minister’s proposal for a European peacekeeping force but other allies, including Poland, looked likely to reject the idea following an emergency meeting in Paris on Monday.

Other countries, including Italy and Norway, are said to be undecided.

Sir Keir scrambled to meet EU counterparts and Nato leaders to thrash out plans to deploy European troops to Ukraine after the US barred the continent from talks with Russia in Saudi Arabia.

The meeting, called by French President Emmanuel Macron, saw Sir Keir joined by leaders from Germany, Italy, Poland, Spain, the Netherlands and Denmark along with the presidents of the European Council and European Commission and the secretary-general of Nato Mark Rutte.

“The whole discussion is moot without both Germany and Poland’s participation,” a diplomatic source said.

Ahead of the talks, José Manuel Albares, Spain’s foreign minister, said: “Nobody is currently considering sending troops to Ukraine. Firstly, because peace is still very far away and for one reason only: Vladimir Putin.”

Poland has ruled itself out of a peacekeeping mission because Warsaw’s troops are committed to protecting Nato’s eastern flank.

Both Hungary and Slovakia are also highly unlikely to join given their governments’ close ties with the Kremlin.

Polish Prime Minister Donald Tusk piled pressure on other European leaders to massively ramp up defense spending, with his country being the top military spender in Nato at 4.7% of gross domestic product

However Sir Keir is unlikely to go beyond his commitment to set out a plan to increase defence spending to a 2.5% share of the economy, despite calls from Mr Trump to hike spending to 5% and Nato chief Mark Rutte’s suggestion allies should spend more than 3%.

The UK currently spends around 2.3% of gross national product on defence.

But more people would oppose than support tax rises to pay for an increase in defence spending, YouGov polling has suggested.

The EU and UK were left reeling over the weekend when top US officials announced they were starting bilateral talks with Russia to end the war in Ukraine — but without participation from Europe and Kyiv.

Sir Keir hopes to be a bridge between Europe and the US when he meets Donald Trump in Washington next week following concerns the US will reduce its defence commitments in the continent.

Speaking ahead of the emergency meeting, Sir Keir said his message to counterparts would be to “step up” and that it was important to have “realistic and credible answers” for how to make any peace agreement last.

He said: “I think that we need to do more. We need to step up in terms of our collective response in Europe, and by that I mean capability. By that, I mean playing our full part when it comes to the defence of the sovereignty of Ukraine if there’s a peace agreement, and, of course, when it comes to funding and training.

“So, on all those fronts, I want the UK and all European allies to step up, and for the UK to play a leading part in that.

US secretary of state Marco Rubio is leading a delegation to Saudi Arabia for talks with Russian officials that will seek an end to the fighting after sidelining Europe.

Ukraine has not been invited to the talks and will not accept the outcome if Kyiv is not involved, Ukrainian President Volodymyr Zelensky said on Monday.

It comes after the US State Department sent a survey to European capitals asking for information on what weapons and peacekeeping troops they could provide to Kyiv after an end to the fighting.

But former army chief Lord Richard Dannatt warned that the UK lacks forces and equipment, adding that putting boots on the ground in Ukraine would come at a “considerable cost”.

He said the Prime Minister has “got to realise that’s going to come at a considerable cost” if he wants to put boots on the ground in Ukraine.

Lord Dannatt added: “If the Prime Minister is serious about wanting to deploy British troops, put boots on the ground in Ukraine as part of a peacekeeping force, he’s got to realise that’s going to come at a considerable cost.

“Frankly, we haven’t got the numbers and we haven’t got the equipment to put a large force onto the ground for an extended period of time at the present moment.

“Now, if Keir Starmer wants to do that, that’s fine. The British Army will always stand up to the plate, but here we go again. We’ve got to have the right number of people with the right equipment and the right training, and start to fund that now.”

Number 10 has not ruled out a vote in Parliament about a future decision to put troops in Ukraine after both Labour and opposition MPs called for one before the UK sends troops to Ukraine.

Liberal Democrat leader Sir Ed Davey said: “In principle there should be a vote in Parliament when troops are deployed.

“I’m confident all sides of the House are likely to agree with the Prime Minister apart from those Trump bootlickers in Reform.”

Zia Yusuf, chairman of Reform, said: “Has anyone else noticed that political leaders start to get really keen to send other people’s children to war zones after their domestic poll ratings collapse?”