Jeremy Corbyn is coming under pressure amid divisions over Labour’s Brexit strategy as leading figures call for the party to back staying in the EU.
Shadow foreign secretary Emily Thornberry said Labour must “say no” to leaving the EU at its party conference.
And deputy leader Tom Watson said it must settle its position as “a Remain party” once and for all this week.
But Unite leader Len McCluskey said anyone who could not support Jeremy Corbyn’s position should stand aside.
He said talk of divisions were “fake news” given that Labour had a policy of giving the public the final say in another referendum which the shadow cabinet could unite around.
The leadership will seek the support of its members for this policy in a vote on Monday. The exact wording of the motion to be debated will be decided later on Sunday.
The party’s NEC, or governing body, has agreed a motion which calls for the party to renegotiate the current terms of exit and then give voters the choice to back the new Brexit deal or to remain in the EU.
Mr Corbyn has persistently refused to be drawn on which way he would campaign in another vote, saying it would depend on the kind of agreement he struck.
But he is under growing pressure to declare his hand from pro-EU figures in the party.