EMILY Thornberry has warned Jeremy Corbyn that Labour could lose almost a third of its vote at the next General Election unless it came out unequivocally behind a pro-Remain policy to keep Britain in the EU.

The Shadow Foreign Secretary spoke as the deep division between the party leader and some of his most senior colleagues was laid bare ahead of what promises to be an impassioned and potentially watershed moment for Labour in its debate on Brexit this afternoon.

Speaking at a fringe event, Ms Thornberry said: “The polling shows we could lose 30 per cent of the Labour vote to the Greens and the Lib Dems unless we are clear about where we stand on Europe.

“I want Jeremy to be in No 10 and the best chance of doing that is to speak truthfully, which is: we as a party are a Remain party.”

The London MP added: “The lesson we should learn from the European elections is that we do need to be clear. We’re an internationalist party, we’re a Socialist party and that it is natural in those circumstances for us to campaign to Remain officially.”

Tom Watson, the deputy leader, at another fringe event echoed the point, declaring: "We are a Remain party. We are a European Party. We are an internationalist party. That is who we are.”

Sadiq Khan, the London Mayor, was even firmer in his call to his party leader to join his colleagues and swing the party fully behind Remain.

He 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."

Richard Leonard, the Scottish Labour leader and a loyal Corbynite, made clear his party would campaign “unambiguously for Remain” and called on the UK leadership for “clarity” on the issue.

But Len McCluskey, the leader of the Unite union, who is said to have Mr Corbyn’s ear, delivered a stark warning to the likes of Ms Thornberry, urging them to “support your leader” and, if they could not, then they should “step aside from the Shadow Cabinet…and they can argue for whatever they want”.

Members of Labour's ruling National Executive Committee voted by 16 to 10 in favour of Mr Corbyn’s position: to delay a decision to back staying in the EU or withdraw until after a general election when a special conference would be held to decide the way ahead.

Last night, talks were going on to composite some 80 motions, calling for the party to back Remain, which will form the basis of the opposing motion in today’s debate.

Pro-Remainers were furious with the leader’s position. Clive Lewis, the Shadow Treasury Minister, said: “It’s the equivalent of saying to an officer in the trenches: ‘What’s the objective?’ And him saying: ‘I don’t know, we will decide once we are on the other side.’”

Earlier, Mr Corbyn played down the resignation of key aide Andrew Fisher, who in a leaked memo denounced the Labour leader's inner circle for its "lack of professionalism, competence and human decency".

The head of policy and the author of the party's last manifesto said he was sick of the "blizzard of lies and excuses" and claimed a "class war" had gripped the upper echelons of the party.

But appearing on BBC One's The Andrew Marr Show, Mr Corbyn described Mr Fisher as a “great colleague, a great friend,” who was leaving to spend more time with his family.

The Labour leader also sought to brush aside the row over the bid to oust Mr Watson as his deputy, claiming the attempt by Momentum to scrap the role was not “specifically” aimed at the Midlands MP.

Mr Corbyn has proposed a review about having two deputy leaders in the future, which would reflect gender and ethnic balance in society; a move agreed to by the NEC.

In a separate development, the Labour leader dismissed talk that he could stand aside early as “wishful thinking” and made clear he would, if elected, serve a full five-year term as PM.

“Of course, why wouldn’t I?” he asked, adding: “I’m looking forward to the job.”

In other developments:

*an opinion poll put the Conservatives 15 points ahead of Labour, 37 to 22;

*Cabinet Office Minister Michael Gove warned that support for the Tory Party would “collapse” if the UK did not leave the EU on October 31 and

*Dominic Raab, the Foreign Secretary, said there would not be a Northern Ireland-only backstop and made clear any Brexit deal would have to have the consent of the people of the province.