Labour will be unable to win a majority in a general election unless it changes its stance on a second referendum, new analysis of polling suggests.

Jeremy Corbyn’s party stands to lose more than 40 per cent of voters who backed Labour in 2017, with support overwhelmingly switching to Remain-backing parties.

The figures come as one of Labour’s shadow Brexit ministers, Jenny Chapman, swung behind a Final Say referendum, having previously been a staunch opponent.

Ms Chapman said she was convinced there was now no alternative way to resolve the crisis. Asked if the Labour leader should announce a shift in policy straight away, she told the BBC: “I think so – I think it should be now.”

The party is braced to lose the Peterborough constituency to Nigel Farage’s Brexit Party in a by-election on Thursday, which is expected to see Liberal Democrats benefit from Remain switchers frustrated by the lack of clarity in Mr Corbyn’s stance.

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Despite calls from senior shadow cabinet members, including deputy leader Tom Watson, to throw the party’s weight behind a second referendum on any Brexit deal, Mr Corbyn has stuck to the policy of supporting a people’s vote only if Labour cannot obtain its preferred EU withdrawal outcome or a general election. And he has kept the party’s campaign focus on issues like austerity, health and public services rather than Europe.

Applying polling responses from more than 15,000 voters over the past month to individual constituencies, data analysis company Focaldata found that Labour would be the biggest party in the new parliament, but might lose more than 40 per cent of its 2017 support.

According to the figures, Jeremy Corbyn’s party would be left well short of a majority and forced to rely on some sort of deal with Remain-backing Liberal Democrats and nationalist parties to form a government, making a Final Say poll all but inevitable.

Focaldata’s analysis for Best for Britain and the Hope Not Hate campaign found that 42 per cent of those who backed Labour in 2017 would desert the party in a new general election, with the vast bulk (30 per cent) going to Remain-backing parties like the Lib Dems, 2 per cent to the Scottish National Party or Plaid Cymru and 10 per cent to the Brexit Party.

Tories would lose more than half of the voters who backed Theresa May in the last general election, with 37 per cent going to Farage’s party, 8 per cent to the Lib Dems and 2 per cent each to Labour and the Greens.

Labour would take 24.8 per cent of the vote, ahead of the Brexit Party on 22.9 per cent, with the Tories trailing in third with 21.7 and Lib Dems on 17.6.

On these figures, Mr Corbyn could find himself at the head of a 251-strong Labour contingent in the Commons, as the largest single party but far from the 326 MPs needed for an absolute majority.

With the Brexit Party taking 135 seats and Tories 132 under these calculations, Lib Dems on 53 and the SNP enjoying a near clean-sweep of 55 out of 59 constituencies north of the border, Mr Corbyn may be forced to seek an anti-Brexit alliance of some form to have any hope of governing.

According to the analysis, prominent Tories like defence secretary Penny Mordaunt, work and pensions secretary, Amber Rudd, and communities secretary, James Brokenshire, could all lose their seats, while leadership favourite Boris Johnson’s Uxbridge and Ruislip would become a marginal.

Best for Britain chief operating officer Naomi Smith said: “The consequences of Labour’s ambivalent policy towards a Final Say on Brexit looks set to hurt them further. They’re expected to lose almost half of their 2017 vote share, according to our analysis.

“The Labour leadership now needs to pick a side, and with them losing three times as many votes to Remain parties as they are to the Brexit Party, it is obvious which position they should take if they want to be in government.”

Nigel Farage says the Brexit Party could 'stun everybody' in a general election

The chief executive of Hope Not Hate, Nick Lowles, said: “This poll reveals a quite shocking surge in a hardline right-wing Brexit Party, led by a divisive and dangerous politician in Nigel Farage.

“We are onto very, very dangerous ground and every mainstream party needs to think very carefully about how to avoid these results coming to pass.”

Mr Lowles said Labour was “bleeding votes” to parties which have taken a clear anti-Brexit stance.

“Labour will lose seats in areas that voted Leave if they continue to lose the support of Labour Remain voters,” he said. “It’s counter-intuitive but it’s the reality.

“The evidence is clear – from this poll, and the recent election results – if Labour wants to win a majority at the next election, it needs to move to keep its Remain voters on board.”