Members of Jeremy Corbyn's top team should either support him on Brexit or "walk away," a senior ally of the labor leader told Sky News.
Union leader Len McCluskey spoke amid a fresh wave of unease over the party's position on the UK's exit from the EU.
A policy statement by Corbyn to the Labor National Executive Committee, if adopted, would mean that the party would go into a general election campaign without a clear stance on which side to back a second referendum.
Emily Thornberry and Sir Keir Starmer in the people's vote march on Saturday
The statement says Labor will close a new deal with Brussels in three months and then put it in another public vote.
His position in that referendum would be established at a special conference after an election.
The NEC ruling angered some party members amid continuing calls from certain sectors for Corbyn to support the rest of the population in a second vote.
Dozens of motions calling for this policy change were submitted to the party's annual conference in Brighton.
But supporters of this move fear that the NEC statement, which has not yet been signed, will end the debate on the issue.
Entangled frustrated to kick me out & # 39; very sad & # 39;
Emily Thornberry and Sir Keir Starmer – two prominent members of Labor's shadow office – spoke in favor of Remain at a second referendum rally on Saturday.
Issuing his warning to the parallel cabinet, McCluskey said the Labor Party "must enter a united election."
He told Sophy Ridge on Sunday: "When we have a policy on Brexit and Jeremy Corbyn makes it clear that this is the policy, that is what the leading members of the parallel cabinet should argue.
"If they think they can't argue for it, because they feel strongly, well, of course they have that right, but they should move away from the shadow cabinet that will become the cabinet and they can argue whatever they want." .
"But politics and my appeal to them, Emily and everyone else, are (support) the leader."
Corbyn asks questions about Watson's plot
Speaking to the Andrew Marr Show of the BBC, Mr. Corbyn defended his position.
"What we said is that we would like to hold a consultation, a special conference of our party, when we receive this offer from the EU, we have this as an option to stay – and I hope to reform -". he said.
"Because I think even those who are strongly in favor of Remain would recognize that the EU needs some reforms."
Undercover health secretary Jonathan Ashworth told Sky News that there was "merit" in the proposed position.
Speaking at the Popular Vote event, Labor spokesman for Brexit, Sir Keir Starmer, said another public vote is the "only way out" of the current impasse.
Sir Keir said he would support Remain if such a vote happens.
Thornberry told the crowd that Labor should campaign to stay in the second referendum.
Jeremy Corbyn defended his position at Brexit
Conservative Party President James Cleverly said: "Jeremy Corbyn cannot lead his own party, much less the country – he cannot even decide on the most important issue facing the country.
"He would delay Brexit by at least 2020 and even more if the EU demands it – Brussels would be in the driver's seat and Corbyn would accept anything from them."
"Only Boris Johnson and the Conservatives will make Brexit until October 31, whatever the circumstances, and will continue to deliver the change that people voted for."