Labour's Brexit civil war deepened today as pro-Remain MPs led by Tom Watson launched a formal campaign to keep Britain in the EU amid open speculation over how much longer Jeremy Corbyn can stay leader.

Mr Watson has launched a website called Labour Remain and he is urging activists to sign a declaration vowing to 'fight, fight and fight again to keep Britain in the EU'.

The deputy leader's declaration states that Labour must become the 'party of Remain' if it is to have any chance of winning the next general election.

But Labour MPs who believe the 2016 EU referendum result must be delivered have warned against the party calling for a second referendum and backing Remain.

They fear that such a move would result in devastating losses to The Brexit Party in Labour's northern heartlands where many supporters voted to Leave the EU.

Mr Corbyn has repeatedly delayed making a final decision on whether Labour should call for a referendum in all circumstances and whether to back Remain.

Meanwhile, a recent poll which put Labour fourth on just 18 per cent of the vote has led to MPs discussing publicly whether he can keep on going as leader.

Mr Corbyn's grip on power has been progressively weakened because of the Brexit row and Mr Watson has piled on even more pressure with his latest push for the party to adopt a pro-Remain stance.

He said: 'Labour values are European values. That's why today I am asking Labour members and supporters to sign a declaration proclaiming support for remaining in the EU and urging the Labour Party to lead the campaign for Remain.'

Jeremy Corbyn, pictured yesterday in Cheshire, has been blamed for the Labour Party's slumping support over his Brexit dithering

Tom Watson continues to push for Labour to fully commit to backing Remain and today he launched a new website to put fresh pressure on Mr Corbyn to change the party's stance

Mr Watson's Labour Remain sign urges activists to sign a pledge stating that Labour should 'fight, fight, and fight again to keep Britain in the EU'

The declaration states that 'Labour must lead the campaign to Remain' and 'as a party of Brexit we will never win again'.

It states: 'As the party of Remain, we will not take every voter with us, but it's the only way that Labour can win, and the only way to keep our country together.

'Because we love Britain, and because we believe in Labour, we will fight, fight and fight again to keep Britain in the EU.

'The next step, which only Labour can deliver, needs to be a public vote on the Brexit fiasco, with Labour leading the way for Remain.'

But Caroline Flint, a Labour MP who has repeatedly stressed the importance of delivering on the EU referendum result, warned against the move.

Asked by the BBC's Andrew Neil if it was now the 'beginning of the end for Mr Corbyn' because of how bad Labour is doing in the polls, Ms Flint replied: 'It's certainly something everybody should be giving some thought to because above us is The Brexit Party and then it's the Tories according to that poll and of course it is really worrying.

'Everyone will jump on the bandwagon, I am sure, in my own party and say this is because we are not supporting a second referendum, this is because we are not Remain enough.

'But when you bear down into the polling what it would show is that in a general election we would lose I think 80 seats to The Brexit Party, 11 to the Lib Dems and 15 to the Tories.'

In the recent YouGov poll just 43 per cent of those who voted Labour in 2017 said they would do so again.

Ms Flint added: 'I think it is difficult for the Labour Party - there's difficulties for the Tories on this - but in all honesty our party is split, our parliamentary party is split on this, and it is a difficult issue across the whole country.

'That is why I believe that when we come back after the recess and if there are more choices to be made it would be better to give a free vote to Labour MPs who want to vote with their conscience on this issue.'

The YouGov poll sent the party into a panic because its 18 per cent rating equalled its joint worst ever performance under Gordon Brown during the financial crisis.

The survey put the party in fourth place after dropping two points in a week - behind the Tories, Brexit Party and Lib Dems.

Mr Corbyn has spent months desperately flannelling over whether to support a referendum with a decision potentially being taken next week.