Vladimir Putin will attack again unless America agrees to underwrite the security of Ukraine after a peace deal has been signed, Sir Keir Starmer has warned.
The Prime Minister also made clear that he was considering deploying British troops as part of a wider European force in Ukraine but it would be a decision "not taken lightly" and relied on continued involvement of US forces in the country.
The issue of the backstop offered to Ukraine will be a key part of talks between Starmer and US President Donald Trump on Thursday.
Speaking to reporters en route to Washington DC, Starmer said: "The reason I say the backstop is so important is that the security guarantee has to be sufficient to deter Putin from coming again because my concern is if there is a ceasefire without a backstop, it will simply give him the opportunity to wait and to come again because his ambition in relation to Ukraine is pretty obvious."
Vladimir Putin will attack again unless America agrees to underwrite the security of Ukraine after a peace deal has been signed, Sir Keir Starmer has warned
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Starmer shrugged off questions over whether Putin will agree to the European peacekeepers plan, saying: "I’m not answering to Putin.
"I’m considering how we preserve peace in Europe and how we get a lasting peace in Ukraine.
"And I’m absolutely convinced that we need a lasting peace, not a ceasefire, and for that to happen we need security guarantees.
"Precisely what that layers up, to what that looks like, is obviously a subject of intense discussion.
"We will play our part and I’ve been clear that we will need a US backstop of some sort.”
Asked whether he would rule out deploying UK troops as part of a peacekeeping force, he said he was "not going to get ahead of myself".
He said: "We’re in a really important moment, we’re three years into the conflict, there are some talks going on now. We all want a peaceful outcome.
"It’s got to be a lasting peace, and that requires us to put in place an effective security guarantee. Exactly what the configuration of that is, exactly what the backstop is, is obviously the subject of intense discussion."
But he made clear that if British troops were to be deployed to Ukraine it would weigh heavily on the PM as it was a decision "not taken lightly".
He said: "I want a lasting peace, and I don't think that will happen if there isn't an effective deterrence to Putin.
"The UK will play its part in that with others but we need a US backstop because I don't think there will be a deterrent to Putin if we don't have one.
"I understand the gravity of the situation but this is about ensuring that we all enjoy peace. It's about the security and defence not just of Ukraine and Europe but our own security and defence."