If a referendum was held today 55 of the 89 most marginal constituencies would support EU membership, up from just 25 which backed Remain in 2016, according to new analysis.
The study by Professor Chris Hanretty from Royal Holloway said Jeremy Corbyn’s path to Downing Street runs through seats that have switched since the Brexit vote.
It also reveals Theresa May’s attempts to gain cross-party support for her Brexit deal in the crunch Commons vote will have been damaged by the fact constituencies represented by Labour MPs have drifted towards Remain.
Writing for HuffPost UK, Dr Alan Wager from the UK in a Changing Europe think-tank said the analysis showed 46 Labour backbenchers represent seats which have switched from Leave to Remain.
“Without significant support from the Labour benches, Theresa May will find it impossible to get her deal through Parliament,” he said.
In a significant intervention on Wednesday, John McDonnell said it now seemed “inevitable” that Labour would end up backing a second referendum if the prime minister’s deal is voted down.
The shadow chancellor’s Hayes and Harlington constituency has seen the fifth largest swing from Leave to Remain in England and Wales. The west London seat voted 59%-41% Leave in 2016, but now would back Remain by 54%-46%.
Jo Johnson, the former Tory minister who quit to demand a referendum on Theresa May’s Brexit deal, will warn on Thursday the prime minister’s plan could “trigger an electoral defeat on the scale of 1997”.
In a speech, Johnson will say: “The Conservative Party’s reputation for economic competence would be undermined by implementing a botched Brexit, especially one that the Government’s own analysis suggests will cause economic harm.
“Brexit is seen as a project driven by the Conservative Party and this half-baked, worst of all worlds Brexit could trigger an electoral defeat on the scale of 1997 or worse, with this “Tory Brexit” label an albatross around our necks for years to come.”
The new research shows a string of marginal seats - calculated as those where a 5% swing in either direction would see it change hands - have been affected by shifts in opinion on Brexit since 2016.
South Swindon, held by Solicitor General Robert Buckland by 2,464 votes, has moved from a 52%-48% Leave majority to a 55%-45% Remain majority.
Nicky Morgan’s Loughborough seat was split 50%-50% at the 2016 referendum. But it would now vote 59%-41% to Remain.
Work and Pensions Secretary Amber Rudd’s seat of Hastings and Rye, which she holds with a majority of just 346, was only 42% Remain at the referendum. But it is now 50%-50%.
Southampton Itchen, the most marginal seat in England and Wales in 2017, is held by Tory MP Royston Smith by just 31 votes. In 2016 it was 60%-40% Leave. It is now 57%-43% Remain.
However, not all seats have moved towards Remain and the complex picture of shifting views on across the country helps to explain Corbyn’s refusal to wholeheartedly embrace a second referendum.
According to the study, 32 of the marginal seats that Labour will hope to gain currently back Leave.
And there are still a number of Leave-supporting seats held by Labour which the Conservative Party will aggressively target.
Wakefield, won by Mary Creagh with 2,176 votes, is still 57% Leave – down from 63% in 2016.
MPs are due to vote on May’s Brexit deal on December 11 and Downing Street had hoped Labour MPs from Leave constituencies could be persuaded to back the government.
But that route to a Commons victory now appears closed, after wavering Labour MPs including Lisa Nandy said they would vote against the deal.
Foreign Secretary Jeremy Hunt’s seat in Surrey has become less Remain-supporting, with backing for Leave shifting up by two points.