The ‘Remain Alliance’ of three anti-Brexit parties says it can win “at least 44” seats and deny Boris Johnson an election win – as Labour condemned all “pacts, coalitions or deals”.

“That would be enough to ensure that Boris Johnson doesn’t have a majority,” said Molly Scott Cato, a Green MEP.

But the initiative – the first pact by parties at a British election for 100 years – was met with a furious attack by John McDonnell, Labour’s shadow chancellor.

“I remember what the Lib Dems did when they were in government. Go and ask any disabled person in this country who went through the brutality of the work capability assessment,” he said, on the campaign trail in Liverpool.

“We will never enter any pacts, coalitions or deals like that ever.”

The Unite to Remain campaign said it had been “rebuffed” by Labour, some of the party’s figures warning they “can’t even be seen to be talking to you or I will be expelled”.

John Curtice, the polling expert, has dismissed the impact of Unite to Remain as “minimal” – arguing it will deprive the Tories of six seats at most – because most anti-Brexit voters are Labour supporters.

Of the 60 targets, not one would have delivered a different result at the last election in 2017 if the Lib Dems, Greens and Welsh Nationalists had fielded a single candidate.

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But Peter Dunphy, the alliance’s election strategist, dismissed those results as irrelevant – because the “massive swings to Labour” two-and-a-half years ago had long since disappeared.

“At least 44 of those 60 seats can be regarded as highly winnable,” he told The Independent, adding that meant it required a less than 10 per cent swing.

A single Remain candidate would see only a “tiny” number of voters switch to the Tories, polling suggested, Mr Dunphy said, adding: “In Conservative-facing seats, that could be critical.”

Across the 60 seats, in England and Wales only, the Lib Dems will stand in 43 constituencies, the Greens in 10 and Plaid Cymru in seven.

The most high-profile target of the pact is Dominic Raab, the hard-Brexit backing foreign secretary, with the Greens standing aside for Lib Dems in his Surrey constituency of Esher and Walton.

Mr Raab romped home with a 39-point lead in 2017, but a new poll offers hope to Jo Swinson’s party, suggesting he is only 9 points ahead – even before the Greens stand aside.

Heidi Allen, the chairwoman of Unite to Remain and the outgoing Lib Dem - and previously Conservative MP in one of the target seats, South Cambridgeshire, hailed an arrangement “unprecedented in modern British political history”.