Image caption Unite general secretary Len McCluskey says he has not had any secret meetings with Momentum

The boss of Britain's biggest union Unite has accused Labour's deputy leader of behaving like a "low-budget remake of the Godfather".

Len McCluskey rejected former flatmate Tom Watson's claims of a "secret" hard-left plan to take over the party.

Mr Watson says Unite have plans to pump cash into left-wing campaign group Momentum, which would "destroy" Labour.

In a Huffington Post blog, Mr McCluskey accused Mr Watson of "sharpening his knife looking for a back to stab".

But Mr Watson hit back, saying his former close friend was resorting to "personal insults rather than arguing his case".

The row follows comments made by Momentum's Jon Lansman, who was reportedly taped saying Unite could affiliate to his group rather than just to Labour.

Unite is the Labour Party's biggest financial backer but Mr Watson fears that if it started supporting Momentum as well as - or instead of - Labour it would threaten the party's "very existence as an electoral force in the land and it needs to stop".

He urged Mr McCluskey to publicly state Unite would not fund Momentum if he was re-elected as Unite leader in the union's on-going election campaign.

Some Labour MPs fear Momentum, which grew out of the campaign to get Jeremy Corbyn elected as leader, is a "party within a party" and is behind attempts to remove them as candidates and install more left-wing alternatives.

Media playback is unsupported on your device Media caption Jeremy Corbyn calls for unity after 'explosive' parliamentary Labour Party meeting

But the group insists it wants to democratise Labour and give ordinary members more of a voice.

Mr McCluskey, who was once a flatmate of Mr Watson, before their spectacular falling out, said the Labour MP's claims were "a complete fabrication" and "there are no plans to fund anybody".

Mr McCluskey said he had "not met Jon Lansman - there have been no secret meetings with anybody about Momentum", and he accused Mr Watson of "a deliberate attempt to sensationalise something in order to influence the outcome of the general secretary election of Unite".

"Really, Tom and the other right-wing Labour MPs would be best keeping their nose out of our business because the truth of the matter is, my members will reject any attempt from outside bodies to influence and to try to take over Unite - it won't work," he said.

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On Monday evening, Labour leader Jeremy Corbyn addressed an "explosive" meeting of the Parliamentary Labour Party, where there was shouting over the leadership's handling of the issue, with one Labour MP referring to Mr Corbyn as "a so-called leader".

Mr Watson was "cheered to the rafters" as he arrived at the gathering, with senior figures in the party challenging claims he had been isolated and condemned at a "robust" earlier meeting of the shadow cabinet, which prompted Mr Corbyn and his deputy to issue a joint statement agreeing to strengthen party unity.

An aide to the Labour leader denied briefing against Mr Watson and said Mr Corbyn's message had been on the need for the party to talk about the issues which affect Labour voters - not internal issues within the Labour Party.

Media playback is unsupported on your device Media caption Labour needs to be "more disciplined" Emily Thornberry tells BBC Newsnight

It is understood the chairman of the Parliamentary Labour Party warned that Momentum was operating as a "party within a party" after hearing a tape of Momentum founder Mr Lansman reportedly urging supporters to change the nature of the Labour party.

One senior peer, who is a veteran of earlier internal battles, said it was "like 1985 all over again", while another senior peer said it was "much worse than that".

In the recordings, which were revealed by The Observer newspaper, Mr Lansman said: "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 stands down, they are able to get a candidate on to the ballot paper to succeed him.

Ahead of Monday's meeting, shadow chancellor John McDonnell said Mr Watson's comments were "disappointing" and suggested he was trying to influence the outcome of the contest for Unite leader - in which incumbent Mr McCluskey - a supporter of Mr Corbyn - is being challenged by Gerard Coyne and Ian Allinson.

The ballot papers for the contest will be sent out later this week, with the result to be announced next month.