His own personal approval ratings have hit rock bottom: a poll last week put it at minus 47 per cent, well below Mr Farage and Kemi Badenoch, the Tory leader.

It comes as speculation mounts that the Prime Minister could face a leadership challenge after May, when Labour is expected to be trounced in local elections.

Possible replacements include Angela Rayner and Andy Burnham, who are both more Left wing.

In an interview with The Economist, Sir Keir revealed he had discussed the rise of populism and the threat to centrist parties with European leaders.

And he admitted that he would be able to “sleep at night” if Ms Badenoch entered No 10 – but not if Mr Farage won the next election.

“If there is a Conservative government, I can sleep at night,” he said. “If there was a Right-wing government in the United Kingdom, that would be a different proposition.”

Reform ‘would tear our country apart’

Friedrich Merz, the German chancellor, has said he is the “last chance” for the centre in Germany, where there has been a surge for the Right-wing AfD party.

Asked whether he saw himself as the “last chance for the centre”, Sir Keir said: “Yes. I think this is the real test of centrist politics.

“I don’t mean Labour versus Tories, because to some extent that’s what we’ve had. It’s a different proposition.

“And we have to show that politics can deliver. We also have to expose the Right for what it is, because the propositions of Reform would tear our country apart.”

The Prime Minister said that despite their commanding position in the opinion polls, there was nothing that Labour could learn from Reform.

Centre-Right has ‘fallen away’

He said an increasing number of people “don’t trust centrist parties to deliver”, and that “we have to address that” by delivering economic growth and improved public services.

The Prime Minister added: “My worry is – and this is how history tells us – is that a Right-wing proposition like that, with fantastical ideas, leaning towards Russia, if it gets into power, it will find that its policies don’t work and it won’t become more progressive, it won’t nudge towards the centre, it’ll be go to the Right, to the Right, to the Right.

“We’ve not faced that in this country ever in this form. And it’s the political fight of our times, and we have to win that political fight.”

Sir Keir said that in Britain, the “centre-Right has sort of fallen away”.

“What’s being tested here in the UK and in France and in Germany is whether mainstream politics, centrist parties can deliver against the challenges of the moment.

“The answer to that is yes, but that to me is where there is the challenge.”