Senior Labour figures are mounting a fightback against pressure for change in the party’s Brexit position, as the deeply-divided shadow cabinet meets amid calls for it to adopt full-throated support for a second referendum.

A leaked briefing paper understood to have been presented to the meeting warns of an “evident risk” that shifting to a more explicitly pro-Remain position would cost the party seats in the Midlands and North of England.

And a tweet from the account of chairman Ian Lavery – which he later said had not been authorized by him or his staff – appeared to suggest that the ultimate aim of those pressing for change was to block Brexit by revoking the UK’s Article 50 process.

A letter to leader Jeremy Corbyn from 26 MPs in mainly Leave-backing constituencies warned that a commitment to a second referendum would be “toxic to our bedrock Labour voters”.

The letter, signed by frontbencher Gloria de Piero and MPs including Stephen Kinnock, Caroline Flint and Lisa Nandy, said that the near-defeat by Nigel Farage’s Brexit Party in the Peterborough by-election was a “stark warning” of the potential risk to the party in Leave-majority seats.

“The strength of the Brexit Party in Labour heartland areas in the European elections revealed a much more potent threat than either the Liberals or Greens present,” they said.

Labour sources played down expectations of a fundamental shift in policy as a result of today’s meeting, which was called as part of a consultation process ordered by Mr Corbyn following its disastrous performance in May’s European elections.

Mr Corbyn said then that Labour was “ready to support a public vote on any deal”, either in the form of a general election or a Final Say referendum.

But deputy leader Tom Watson has been urging him to go further and commit the party to campaign actively for a referendum and to support Remain if one is called.

Mr Watson wants an emergency vote or special conference to authorize a change in policy before the summer, warning that it will be “too late” to stop a no-deal Brexit on 31 October if Labour waits until its annual conference in Brighton in September.

While the autumn conference is Labour’s official policy-making body, a party source confirmed that it is possible to set out a new position “at any time”.

Senior shadow cabinet figures including Mr Lavery and Jon Trickett have successfully pressed Mr Corbyn to resist calls for out-and-out support for a referendum and Remain, warning that it would risk a collapse in support among Leave-voters who make up more than half of the electorate in a majority of Labour-held seats.

But Mr Watson, shadow foreign secretary Emily Thornberry and shadow Brexit secretary Sir Keir Starmer have pushed for stronger backing for a referendum, pointing to the scale of support for Remain among Labour members and voters. After Ms Thornberry was pulled from an expected appearance in the Commons Theresa May joked at prime minister’s questions today that she had been hauled off for “re-education”.

The leaked document for today’s meeting states that “it is not obvious, from the evidence of local elections and Peterborough (by-election) that a more pro-Remain position from Labour would in itself win back voters currently lost to Liberal Democrats – or in a numerical enough way that would offset Leave voters in many of the key marginals”.

And it adds: “It remains the case that there are more target and defensive seats in the Midlands and North of England which voted Leave.

“The recent elections don’t suggest any change to this basic arithmetic, given the geographical distribution of Leave and Remain voters.

“There is an evident risk that shifting to a more explicitly pro-Remain position would leave us vulnerable in seats we need to hold or win without enough potential seat gains in winnable Remain majority areas.”

A tweet sent from Mr Lavery’s account earlier today stated: “Please understand there (sic) position really is to head for to (sic) revoking A50.”