Labour would be trounced by the Liberal Democrats in a general election unless it changes its Brexit stance, according to a new poll.

Almost four in 10 people who voted for Jeremy Corbyn’s party at the 2017 general election would switch their support to the Lib Dems if there was an election later this year and Labour maintained its current Brexit stance, the YouGov survey found.

The finding is likely to add to pressure on Mr Corbyn to agree to fully support another referendum on any Brexit deal, and to commit Labour to campaigning to remain in the EU.

The poll asked voters who they would support in a general election in which Boris Johnson led the Conservatives and promised to take Britain out of the EU on 31 October with or without a deal, Labour maintained its current position in support of a softer Brexit, and the Liberal Democrats under Jo Swinson’s leadership campaigned for Remain. Ms Swinson is currently vying with Sir Ed Davey to take over the leadership from Sir Vince Cable.

It found that 30 per cent would vote Liberal Democrat, 24 per cent would support the Conservatives and 19 per cent would back Nigel Farage’s Brexit Party, once people who say they don’t know are excluded. Labour would finish in fourth place, with just 17 per cent of the vote.

Labour’s existing vote would be decimated, with 62 per cent of voters who backed it in 2017 switching their support to other parties. Thirty-nine per cent of 2017 Labour votes would back the Lib Dems – more than half of the total number of people that would desert Mr Corbyn’s party.

In this scenario, Labour would win the support of just 21 per cent of Remain voters, while the Liberal Democrats would sweep up 52 per cent of them.

If Mr Corbyn opposed Brexit and fully endorsed another referendum, however, Labour would finish joint second, tied with the Lib Dems on 22 per cent. The Tories would top the poll, with 26 per cent of the vote.

However, a quarter of 2017 Labour voters would still back the Liberal Democrats.

Only a Labour leader seen as more anti-Brexit, such as Sir Keir Starmer, the shadow Brexit secretary, would effectively win back voters from the Liberal Democrats, the poll found,

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Marcus Roberts, YouGov’s director of international projects, said: “It’s not as easy as Remainers might hope to say that a shift to Brexit opposition automatically brings back Labour’s post-2017 defectors. Indeed, such a shift could lead to the Remain vote being split equally between Labour and the Liberal Democrats, with the result that the Conservatives would enjoy their strongest showing.”

On what it would take for Labour to win a general election, he said: “Possibly a more enthusiastically Remain leader, such as Sir Keir Starmer, opposing Brexit and supporting a second referendum. But it’s worth noting that this leads to a win only by the narrowest of margins – just one per cent.”

In this scenario, the majority of Labour’s 2017 voters say they would remain loyal – 58 per cent, as well as pulling in 20 per cent of Lib Dem voters and 6 per cent of Green.

He added: “Where under Corbyn, Remain voters were still more likely to vote Lib Dem even if Labour stood against Brexit and for a new referendum, under Starmer, Remainers are more likely to say they would vote Labour than Lib Dem.

“What’s more, this is the only scenario in which a plurality of lost Labour voters would return to the party (35 per cent), while 31 per cent would still vote for the Lib Dems instead.”

Labour declined to comment.

The poll was published ahead of a demonstration in Cheltenham in favour of a fresh referendum.

The “Let Us Be Heard” rally, one of many taking place around the country this summer, will hear from speakers including Alex Chalk, the local Conservative MP.

Mr Chalk has not endorsed a Final Say vote but abstained on the issue when parliament held “indicative votes” on various Brexit outcomes earlier this year.