Sir Keir Starmer’s premiership would be over if Labour’s Caerphilly by-election wipeout is repeated at next May’s local elections, Labour MPs have admitted.

Labour slumped to just 3,713 votes (11 per cent) in the Valleys town on Thursday – a backslide of a staggering 34.9 per cent compared to 2021.

With Reform UK surging from practically a standing start to second place at 36 per cent of the vote, fears are now rising that Sir Keir’s party could permanently slip to third, west of the border.

As the exit poll came in on Thursday, party sources branded the election a “defeat for Starmerism”.

If Caerphilly’s catastrophe is repeated at the May 7 local elections next year, Sir Keir’s time could be up.

“If the national vote across Wales in May is even in the ballpark of Caerphilly, there’s absolutely no way we can carry on like we are after that,” one senior backbencher told The Guardian.

They continued: “Obviously, parties in government struggle in byelections, but when your vote collapses to that extent, someone has got to take responsibility for it.

“Everyone’s miserable, everyone’s despondent. It’s not as if there is an obvious sort of answer or successor to get us out of this mess.

“But we’re all pretty clear, we can’t just carry on as we are.”

More Labour MPs have condemned the party’s “totally incoherent” strategy.

It’s a similar story in the Lords. Peers met No10 chief of staff Morgan McSweeney this week – one told HuffPost the talks a “car crash in slow motion”.

The May 7 locals – if they still go ahead – will see more than 1,600 council seats contested across England, including in London.

Wales and Scotland will also vote for their respective parliaments.

In both countries, opinion polls show that voters would pick Plaid Cymru or the SNP, then Reform, then Labour.

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Labour MPs now fear that tactical voting for nationalist parties could crush their party entirely.

Nigel Farage agrees. He said on Friday: “The Senedd elections next year are a two-horse race between Reform UK and Plaid Cymru.”

“Our strategy is based on the idea we can unify progressive voters around a ‘stop Reform’ campaign,” one Labour MP told The Guardian.

“That works so long as you remain the most viable progressive party. But if people start to think that it’s maybe another party, then suddenly that tactical voting starts to work against Labour and you get punished really badly.”

Another fumed: “The loss of Caerphilly is the direct result of the party losing touch with its own voters.”

Welsh First Minister Baroness Morgan said the by-election was fought “in the midst of difficult headwinds nationally” in an apparent swipe at the PM.

“Welsh Labour has heard the frustration on doorsteps in Caerphilly that the need to feel change in people’s lives has not been quick enough,” she added.

“We take our share of the responsibility for this result. We are listening, we are learning the lessons, and we will come back stronger.”