Labour descended into a state of civil war last night amid accusations of a hard left "plot" to take over the party as the Conservatives extended their huge lead in the polls.
Tom Watson, Labour's Deputy Leader, warned that the future of the party was at risk over claims that a campaign group which backs Jeremy Corbyn is planning to officially link with the union.
Mr Corbyn was heckled at an "explosive" meeting of the Parliamentary Labour Party (PLP) on Monday night, which saw a heated row over the supposed left-wing takeover plot.
The Labour leader later hit back, accusing MPs of "navel-gazing". In a video message released on social media he said: "Sometimes spirits in the Labour Party can run high. Today has been one of those days. That's because we're a passionate party."
One of his own MPs Neil Coyle responded by suggesting Mr Corbyn was attempting to divert attention from "uncomfortable" questions about the Copeland by-election, which saw Labour suffer a humiliating defeat.
Mr Watson's intervention prompted a furious response from allies of Jeremy Corbyn who accused him of "interfering" in the Unite leadership election in a bid to oust Len McCluskey.
John McDonnell, the shadow chancellor, said that he was "disappointed" with Mr Watson's intervention while other members of the shadow cabinet turned on him during a tense meeting.
Labour's turmoil came as an ICM survey for The Guardian found the Tories received approval ratings of 45 per cent, up one point since two weeks ago, while Labour has dropped back two points to 26 per cent.
Despite the fact the poll was carried out in the immediate aftermath of the Government's U-turn on increasing National Insurance payments, the Prime Minister and Chancellor were rated the best team to manage the economy.
They received 44 per cent of the vote, while their Labour opposite numbers Jeremy Corbyn and John McDonnell took just 11 per cent.
On Monday Mr Waston called on Mr Corbyn to "deal with" his supporters after Jon Lansman, the Momentum leader, was secretly recorded revealing that it plans to officially link with Unite.
The move would give the grassroots organisation access to huge funding and institutional support.
Mr Watson told the BBC's Radio 4 Today programme this was now a "battle for the future existence" of the party.
But Mr McDonnell said Mr Watson's comments were "disappointing" and accused him of interfering in the union's leadership contest.
He told the BBC: "This is not civil war...What he is trying to do is influence the election of the general secretary of Unite and he has dragged the Labour Party into this, completely unnecessarily.
"It's all about Tom and the internal battle he is trying to wage within Unite. Dragging the party into this is disappointing."
Ms Thornberry said Mr Watson should not have made his concerns about Momentum and Unite public, telling BBC Newsnight: "I think that it is important that internal discussions and fights within the Labour Party are done privately, we do not need to discuss them on the media.
"It seems to me that these are things that we can discuss internally."
Mr Corbyn and Mr Watson have issued a joint statement yesterday following shadow cabinet 'away day' in central London.
They agreed to strengthen party unity after the in-fighting and said "no one speaks for the leadership except the leadership themselves and their spokespeople."
According to sources present the away day, Mr Watson was criticised by his colleagues in the shadow cabinet for publicly hitting out at Mr Lansman.
He was told that “this kind of intervention is damaging in elections – this behaviour does us no good”.
In a recording obtained by The Observer Mr Lansman tells a Momentum meeting: "Assuming that Len McCluskey wins the general secretaryship, which I think he will, Unite will affiliate to Momentum and will fully participate in Momentum, as will the CWU."
He went on to tell activists it was "absolutely crucial" that they secured a change to the party's rules to ensure that whenever Mr Corbyn stood down, they were able to get a candidate on to the ballot paper to succeed him.
Unite’s acting general secretary Gail Cartmail said: “Tom Watson has made claims about Unite and its general secretary Len McCluskey which are entirely inaccurate.
“As Unite has made it clear it is exclusively for our executive council to determine which organisations we affiliate to. There are no plans for Unite to affiliate to Momentum. For the record, Len McCluskey has never met Jon Lansman to discuss this or any other matter.
“It is extraordinary that the deputy leader of the Labour party should interfere in Unite’s democracy in this way."