Union boss Len McCluskey spoke out amid a fresh bout of disquiet over the party’s stance on the UK’s departure from the EU.
A policy statement put forward by Mr Corbyn to Labour’s National Executive Committee, if adopted, would mean the party would head into a general election campaign without a clear position on which side it would support in a second referendum.
The statement says Labour would strike a new deal with Brussels within three months and then put it to another public vote.
Its position in that referendum would be settled in a special conference after an election.
The NEC move has angered some within the party, amid continuing calls from certain quarters for Mr Corbyn to back the Remain side in a second vote.
Scores of motions calling for such a policy shift have been submitted to the party’s annual conference in Brighton.
But proponents of such a move fear the NEC statement, which has not yet been signed off, would shut down debate on the topic.
Emily Thornberry and Sir Keir Starmer – two prominent members of Labour’s shadow cabinet – spoke out in favour of Remain at a second referendum rally on Saturday.
Issuing his warning to the shadow cabinet, Mr McCluskey said Labour “must go into an election united”.
He told Sophy Ridge on Sunday: “When we have a policy on Brexit and Jeremy Corbyn makes it clear that is the policy, then that’s what leading members of the shadow cabinet should argue for.
“If they find that they can’t argue for it because they feel strongly, well, of course they have that right, but they should step aside, step aside from the shadow cabinet which will become the cabinet and they can argue whatever they want.
“But the policy and my appeal to them, to Emily and anyone else, [is] support the leader.”
Speaking to the BBC’s Andrew Marr Show, Mr Corbyn defended his position.
“What we have said is that we would want to hold a consultation, a special conference of our party at the point that we have got this offer from the EU, we’ve got this as a remain – and hopefully reform – option,” he said.
“Because I do think even those that are strongly in favour of Remain would recognise the EU needs to have some reforms.”
Labour’s shadow health secretary Jonathan Ashworth told Sky News there was “merit” to the proposed position.
But a string of top figures have spoken out in favour of Remain, despite Mr McCluskey’s warning.
Ms Thornberry told a conference fringe event that it was not disloyal to push for the party to back Remain.
Shadow foreign secretary Emily Thornberry told a rally on the conference fringe that it was not disloyal to push for the party to lead the remain campaign in a second Brexit referendum.
“Whatever the proposed terms of the departure are, and whichever government proposes them, we must demand a second referendum, we must demand that the public has a final say and we must not just campaign to remain but we must lead the campaign to remain,” she said.
Ms Thornberry added: “That doesn’t mean that we are not socialists or that we are disloyal to Jeremy Corbyn”.
London mayor Sadiq Khan called for Labour to whip its MPs to campaign for staying in the EU.
Speaking at a New Statesman event, Mr Khan said: “We are at a vital crossroads, neutrality is not an option.
“The party should be unequivocally pro-Remain. All Labour MPs should be whipped to campaign for Remain.”
Nia Griffiths, Labour’s shadow defence secretary, became the latest figure to come out in favour of another referendum.
Speaking today at a rally, she told crowds: “It’s not just no deal that would damage our country. I believe that any Brexit will leave us worse off.
“That’s why I am delighted that it is Labour party policy to give the public the final say on any deal, and that’s why I will be campaigning to Remain in all circumstances.”
Conservative Party chairman James Cleverly said: “Jeremy Corbyn can’t lead his own party, let alone the country – he can’t even make up his mind on the most important issue facing the country.
“He would delay Brexit until at least 2020 and even longer if the EU demand it – Brussels would be in the driving seat and Corbyn would accept anything from them.
“Only Boris Johnson and the Conservatives will get Brexit done by 31 October, whatever the circumstances, and get on with delivering the change that people voted for.”