Labour MPs in Leave-supporting constituencies have written to Jeremy Corbyn to urge him not to go “full Remain” as the party considers its position on another Brexit referendum.
The shadow cabinet is currently discussing whether to soften Labour’s stance on a further public vote.
Labour has promised one if it cannot get its own deal with the EU passed by MPs or if there is no general election.
Deputy leader Tom Watson says such an “ambiguous” stance is harming Labour.
In a speech on Monday, he called for Labour to back another public vote in all circumstances and wholeheartedly campaign to Remain.
He said its apparent equivocation over the issue had seen its lose votes to the Lib Dems and other unashamedly pro-Remain parties in last month’s European elections.
He warned Labour must adopt a stronger position quickly before the election of a new prime minister who could be prepared to take the UK out of the EU without a deal on 31 October.
The BBC’s political correspondent Iain Watson said the MPs’ letter reflected the ongoing tensions within the party on the issue.
Signatories to Wednesday’s letter include Caroline Flint, Lisa Nandy, Stephen Kinnock and Dan Jarvis.
They say “a commitment to a second referendum would be toxic to our bedrock Labour voters, driving a wedge between them and our party”.
A large number of Labour MPs believe any deal on the terms of the UK’s withdrawal needs to be put to a public vote, with the option to remain in the EU on the ballot paper.
Those backing this position include shadow Brexit secretary Sir Keir Starmer and shadow foreign secretary Emily Thornberry.
But Mr Corbyn has always been more cautious. Despite Labour coming third in the European elections, he said that another referendum was still “some way off”.
And MPs in Brexit-backing constituencies, including the party chairman Ian Lavery, have warned another referendum would be a betrayal of the 17.4 million people who voted to leave the EU in the 2016 referendum.