Boris Johnson would see his party lose up to 50 seats at the next general election if Labour joined forces with the pro-Remain parties, according to analysis that is likely to lead to fresh calls for Jeremy Corbyn to open the door to an electoral pact.

While a so-called “Remain alliance” between the smaller pro-EU parties would only affect the outcome in a maximum of around 25 Tory-held seats, the Conservatives would lose almost double that number of constituencies if Labour agreed to be involved, according to polling expert Professor John Curtice.

Such a result would make it all but impossible for the Tories to assemble a pro-Brexit majority in the Commons and would instead raise the prospect of Labour governing in coalition with one of the other parties.

The analysis comes as Mr Johnson was accused of committing a “constitutional outrage” after he asked the Queen to suspend parliament for five weeks until 14 October. Opposition leaders believe the move is an attempt to stop them from blocking a no-deal Brexit.

The findings will pile pressure on Mr Corbyn to agree not to field Labour candidates in seats where there is a two-horse race between the Conservatives and another party – in most cases the Liberal Democrats. Labour has so far dismissed the suggestion.

Writing exclusively for The Independent, Professor Curtice said that Remain voters in England are split between Labour (34 per cent) and the Liberal Democrats (29 per cent) in England, while pro-EU voters in Scotland mostly support the SNP.

In contrast, the pro-Brexit vote is more united, with half of all Leave voters saying they would back the Conservatives in an early general election.

There is mounting speculation of a general election this autumn, and reports – denied by Downing Street – that the prime minister could call a poll for as early as 17 October.

Professor Curtice said the Conservatives would perform well in a snap election if they can take votes away from the Brexit Party while benefiting from Remain voters being split between several more pro-EU parties.

Support for Nigel Farage’s new outfit has fallen by around 6 per cent since Mr Johnson entered No 10, with almost all of it switching to the Conservatives. If the prime minister is able to deliver Brexit, around half of all voters who currently back the Brexit Party say they would instead back the Tories.

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However, the University of Strathclyde professor said the Conservatives’ current poll lead would not be enough to give them a Commons majority and would likely result in the Tories having fewer seats than the 318 won in 2017.

The Liberal Democrats, Green Party, Plaid Cymru and The Independent Group for Change formed a “Remain alliance” in the recent Brecon and Radnorshire by-election, resulting in victory for the Lib Dems at the expense of the Conservatives.

Efforts are under way to turn this into a formal pact at a national level to maximise the chances of electing pro-EU MPs.

But Professor Curtice warned that such an alliance would only have an impact in a small number of seats.

He said that pro-EU voters voting tactically for the party best placed to beat the Conservatives in their area could cost the Tories up to around 25 seats, in the unlikely event that Liberal Democrat voters in Labour-Tory marginals were happy to switch their support wholesale to Labour.

But if Labour joined the alliance and the other parties agreed not to field candidates in Labour-Tory marginals, the Conservatives would lose up to 50 seats, most of them to Mr Corbyn’s party.

Professor Curtice said: “A pact between just these three parties in which only the best placed of them stood in any Conservative-held seat in England or Wales would only likely make a difference in a handful of seats. It is far from being a sure-fire way of ensuring Boris Johnson’s defeat.

“Meanwhile, given the intensity of the debate about Scottish independence, it seems unlikely that it would be possible to form any pact involving the SNP. In any event, the nationalists would seem well set to claim most of the spoils north of the border anyway.

“What, however, could potentially make a difference is a pact that involved Labour.

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“Even if we take into account the likelihood that some of their supporters would stay at home or spoil their ballot, a pact in which the Liberal Democrats, Green and Plaid Cymru stood down in marginal seats that Labour were defending against a Conservative challenge or were trying to wrest away from the Conservatives could potentially cost Boris Johnson up to 50 seats, most of them to Labour.”

However, Labour has given no indication that it is open to forming an alliance with the anti-Brexit parties, and Professor Curtice said the smaller parties would need to weigh up the fact that any pact would likely benefit Labour most.