JEREMY Corbyn has suffered a major blow to his Brexit strategy just hours before a key debate and vote at the Labour conference after Unison, Britain’s biggest trade union, came out for Remain.

Most unions, which have 50 per cent of the conference vote, had been expected to rally behind the Labour leader’s proposal to “delay and decide” on the way forward on Brexit, but Unison, the public services union, has come out in support of campaigning for Remain in all circumstances.

One Unison source said: "This is about the national interest; it's far bigger than what is best for the Labour Party."

Len McCluskey, the leader of Unite, Labour’s largest donor, pleaded with delegates to back Mr Corbyn, telling conference: "I implore you, please give Jeremy the support he needs later so that Prime Minister Corbyn can lead us to a bright new dawn."

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The Labour leader has called for a neutral position going into the expected forthcoming General Election, saying that he would negotiate a new Brexit deal with Brussels which would then be put to a referendum.

Under his approach the party's position on how to campaign would not be decided until a special conference, after the expected general election.

But delegates in Brighton will vote this afternoon on whether the party should decide now to campaign to stay in the EU, even if that means rejecting a deal Mr Corbyn had negotiated with the EU.

John McDonnell, who has said he will campaign for a Remain vote in the promised referendum, said the process put forward by the party leader was "logical" and declared: “There isn't any war in the Labour Party."

In his keynote speech to conference, the Shadow Chancellor said: “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.”

Earlier, he told BBC Radio 4’s Today programme that it was very difficult for people to determine their position in advance of knowing the detail of a Labour deal.

“But my view because I campaigned for Remain, I can't see at the moment a better deal being achieved,” he explained.

Shadow Cabinet ministers including Emily Thornberry and Tom Watson have called for the party to back a Remain vote now rather than wait for a special conference after the election.

Their actions led to Mr McCluskey suggesting they should either get in line or "step aside" from their Shadow Cabinet roles.

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Mr McDonnell told Sky News: "Len is being Len. We are working together as a party to make sure the people have a choice and the people will decide."

The result of the Brexit vote will hinge on whether the unions decide to back Mr Corbyn's position, which was set out in a statement from the party's ruling National Executive Committee.

Aside Unison, most other unions are expected to support the NEC statement.

Mr Corbyn's proposed way forward was emailed around the NEC and endorsed without a formal meeting on Saturday, despite opposition from some members.

Jon Lansman, boss of the Corbyn-supporting Momentum campaign group and an NEC member, denounced the process as a "travesty".

He said: "There was no meeting, no discussion, no consultation with the membership. On one of the biggest issues of the day, this is a travesty. Across the membership there are many different views on Brexit, and on conference floor members should feel free to vote with their conscience."

Before the Brexit showdown, Mr McDonnell was using his keynote speech to confirm plans for Labour to fund free personal care for elderly people in England, emulating the position in Scotland.

The pledge, which would cost an estimated £6 billion a year in 2020/21, would more than double the number of people receiving state-funded support, Labour said.

In a rally on Sunday night, the Shadow Chancellor indicated that a pledge to reform the social security system, including scrapping Universal Credit, would be included in Labour's first Queen's Speech.

He said there had to be a proper social security "safety net," adding: "That has to be on the basis of enabling people to have a decent quality of life with an adequate income. That has to mean getting rid of the bloody Universal Credit."