The proportion of Leave supporters who would abandon the prime minister if the UK is still in the EU – threatening him with a general election defeat – has more than halved in just one month, it has found.

The Conservatives had been on course to lose 9 per cent of the vote they would have enjoyed at an election, but would now lose only four points and remain the largest party in the Commons.

The poll, for the group Represent Us, will add to Labour jitters that Jeremy Corbyn’s backing for an election as soon as a no-deal Brexit has been blocked threatens the party with a shattering defeat.

As The Independent revealed, the party’s MPs are bombarding the whips’ office with demands for their leader to support a Final Say referendum first.

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The survey, carried out by ICM Unlimited, suggests Mr Johnson is winning his battle to convince voters that he should not be blamed for any Brexit delay beyond 31 October.

In recent weeks, the prime minister has ramped up attacks on MPs who passed what he branded the “Surrender Act” – to avert a crash-out Brexit – and turned on the Supreme Court judges who declared his shutdown of parliament illegal.

Now the poll, of 2,013 people, carried out last weekend, has found the Tories would still beat Labour, by 31 to 28 per cent, even if Brexit has been delayed – and Mr Johnson has broken his promise.

That is because only 4 per cent – not the 9 per cent who said they would last month – of angry Conservative voters would switch to Nigel Farage’s Brexit Party, according to the poll.

Mike Buckley, director of the Labour for a Public Vote organisation, said the poll should be a wake-up call for those wanting “an end to Brexit and the election of a Labour government”.

“The story of these polls is clear – the best way to achieve both ends is for Labour to back a referendum first, in which it must campaign for Remain, and only then to back a general election,” he said.

Richard Tunnicliffe, of Represent Us, which campaigns against a hard Brexit, said: “These polls show that Johnson’s strategy is starting to work and that Leaver voters are moving towards forgiving him if he asks for an extension.

“This makes a general election dangerous for Labour, so perhaps it’s time for the leadership to listen to the growing calls in their ranks to sort Brexit first via a referendum.”

Those backers include Tom Watson, the deputy leader, Emily Thornberry, the shadow foreign secretary, and – privately – shadow chancellor John McDonnell and Keir Starmer, the Brexit spokesperson.

An extension to the Article 50 process still looks inevitable, despite the apparent breakthrough in talks between Mr Johnson and Leo Varadkar, the Irish premier, on Friday.

Even if a deal is agreed, it is too late for parliament to approve it in full by Halloween – and if the talks fail, or the deal is defeated at Westminster, the prime minister is likely to resume his demands for an election.