Jeremy Corbyn is facing a showdown at the Labour conference as he aims to keep the party's Brexit position neutral until after a general election.
But he's facing opposition from grassroots activists and members of his top team, who are arguing for the party to campaign to remain in the EU in all circumstances.
Shadow foreign secretary Emily Thornberry used her speech to urge members to call for the party to throw its weight behind Remain - in defiance of Mr Corbyn's stance.
Activists in Brighton could crush Mr Corbyn's hopes of maintaining a neutral position when they vote on whether the party should campaign to stay in the European Union.
Two separate motions will be put to a vote at the conference on Monday, one calling for Labour to back Remain and another endorsing a policy of neutrality - the latter being Mr Corbyn's preferred choice.
For Mr Corbyn to succeed in his plan, it is likely he will need strong support from a majority of trade unions.
While he has the backing of Unite leader Len McCluskey, head of the UK's biggest trade union, he has lost the support of Unison and Community Union, which are expected to back the Remain motion.
Mr McCluskey, addressing the Labour Party Conference, said: "Let me say here that Jeremy Corbyn is a thousand times right in trying to speak to our whole country at this time of crisis."
He said people should not be defined as Leavers or Remainers, telling delegates: "I implore you, please give Jeremy the support he needs later, so that Prime Minister Corbyn can lead us to a bright new dawn."
Labour's shadow Brexit secretary Sir Keir Starmer - an outspoken Remain supporter - in his keynote speech told Labour members a referendum with an option to leave should be offered to the people.
"Conference, you know where I stand on the question of Remain: I’ve said many times that I will campaign for it, but I profoundly respect those who take a different view. And Conference, let’s go into this with our eyes open."
The chair of the Labour Party told ITV News he believed the vote could go either way and he said the result could "shape the future of the nation".
Ian Lavery MP said: "It really is on a knife-edge and this could shape the general election - it could shape the future of this nation."
He added: "Unity is what we need in the Labour Party at this moment in time."
Another blow to Mr Corbyn's hopes of maintaining a neutral position was dealt by London Mayor Sadiq Khan, who said on Twitter he'd had "really positive" meetings with union leaders.
The tweet suggests leaders of TSSA, Usdaw and Unison may be moving toward his position, which he outlined on Monday when he said it would be "ridiculous" to remain neutral.
Ahead of the votes, Mr Corbyn became angry with members of the media who had gathered around a stall he was attending.
He shouted at reporters: "Can I just say to our friends from the media, this is our conference, these are our stalls, your behaviour - pushing past people, pushing people over, pushing past people who legitimately want to visit stalls, is totally unacceptable."
Mr Corbyn may have been venting his frustration over the party’s deep split on Brexit policy.
ITV News Political Editor Robert Peston reports Mr Corbyn is relying on the support of unions to defeat any vote forcing Labour towards a pro-remain stance.
But as Paul Brand says, wavering union support means "Labour may be on the brink of becoming an unambiguously pro-Remain party in all circumstances this afternoon."
Shadow foreign secretary Ms Thornberry was applauded during her speech at the conference after restating her desire to campaign for remain in any second Brexit referendum.
She said: "With your endorsement today, conference, with the instructions that I hope you give us today, I believe we must strive day and night, whatever it takes, to keep Britain in the European Union."
But a statement by Labour’s ruling National Executive Committee – pushed by Mr Corbyn – says the party should only decide how to campaign in a referendum at a special conference held after a general election.
The lengthy “compositing” meeting to agree the wording of motions followed a day of chaos over Labour’s Brexit stance.
Mr Corbyn’s NEC statement was emailed round the body and endorsed without a formal meeting, despite opposition from some members.
Shadow chancellor John McDonnell also delivered his keynote speech ahead of the votes.
Before getting on stage he told ITV News he believes Mr Corbyn's strategy to only pick a side after negotiating a deal with the EU, "is a logical sequence".
"People have differences of views, of course there are high emotions around this issue because it's an important issue but at the end of the day we respect each other's views and we'll come to a democratic decision."
In his speech, he told Labour members it is right for the final Brexit decision to be made by the people.
"That’s why we aim to trust the people in having the final say on Brexit. A deal or remain.
"Some of you will know I have said I will campaign for Remain. But let me make it clear that I profoundly respect those who support a genuine alternative.
"In our debates today, I want us to demonstrate in the respect we show each other and how we bring our party together just how we can also bring the country together again."