May cost it the election

necessarily through winning a large number of Conservative Leave-voting constituencies.

“Conversely, the Conservatives’ chances of remaining in office depend on not alienating solid Leave voters in these key marginals.

“If Labour backs Leave, then Tory-held marginals would be winnable, and Labour needs them to form a majority.

“But if Labour were to reverse its strategy and back Remain, not only would it place the Tory-held marginals it needs out of reach, but it would risk losing vulnerable Labour-held marginals in Leave-voting areas.

“The most likely outcome of backing Remain would be to give the Tories an overall majority.” Dr Johnson rejected the argument that Labour could afford to lose Leave-voting constituencies.

The impact of Brexit on the next election is the focus of intense discussion.

Last year anti-Brexit group Best For Britain claimed a majority of voters in every seat Labour won in 2017 now back a “people’s vote” and that this was the case in “every seat held by the Conservatives with a majority of 5,000 or less”.

Labour leader Jeremy Corbyn is under pressure from within the party to back a second referendum.

Last week the party suffered a major blow when eight MPs left to form The Independent Group, with Dudley North MP Ian Austin also quitting.