Civil war has broken out within the Labour Party after Shadow Chancellor John McDonnell launched a public attack on Tom Watson, the party's Deputy Leader and MP for West Bromwich East.

Mr Watson says there is a "secret plan" by the hard left to take over Labour - and warned: "I regard this as a battle for the future existence of the Labour Party ."

But Mr McDonnell, Labour's Shadow Chancellor and a close ally of party leader Jeremy Corbyn, has accused Mr Watson of dragging Labour into a row in an attempt to influence the leadership election currently taking place within the Unite trade union.

Unite General Secretary Len McCluskey faces a challenge from Gerard Coyne, the union's West Midlands regional organiser.

Attacking Mr Watson, Mr McDonnell said: "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."

Deputy leader of the Labour Party Tom Watson
Deputy leader of the Labour Party Tom Watson

The Shadow Chancellor told the BBC: "It's all about Tom and the internal battle he is trying to wage within Unite. Dragging the party into this is disappointing."

It follows reports that Mr McCluskey is planning to affiliate the union to Momentum, a grassroots campaign of activists supporting Mr Corbyn.

Momentum founder Jon Lansman was recorded telling supporters he expected the union - together with the Communication Workers Union (CWU) - to affiliate to Momentum if Mr McCluskey wins his battle for re-election as Unite general secretary.

Critics of Mr McCluskey say it would allow the Momentum, which they regard as a hard-left group, to get money and other support from the union.

And Mr Watson is leading the fight against the proposal - and has said he will ask Mr Corbyn what he knows about it.

Labour Leader Jeremy Corbyn, right, and Shadow Chancellor John McDonnell.
Labour Leader Jeremy Corbyn, right, and Shadow Chancellor John McDonnell.

Mr Watson confronted Mr Lansman directly on Twitter, telling him: "You've revealed your plan. If you succeed you will destroy the Labour Party as an electoral force. So you have to be stopped."

And he told BBC Radio 4's Today programme that he would ask Mr Corbyn what he knows, saying: "They [Momentum] organised for him, but I'm not sure if Jeremy knows there's a secret plan. I'll be raising it with him today.

"I'm going to talk to him about it, I don't know what he knows and doesn't know.

"I'm the deputy leader and I didn't know about it until I heard this recording. And I regard this as a battle for the future existence of the Labour Party. This is high stakes.

"What Jon Lansman has outlined is a plan with Len McCluskey, the leader of Unite, to take control of the Labour Party."

Mr Watson said he wanted the Labour leader to "deal with" Mr Lansman.

"The significance of this is that we have never seen the biggest union organising a political faction within the Labour Party with the tacit approval of the leadership," Mr Watson said.

Unite West Midlands regional secretary Gerard Coyne

Meanwhile, Mr Coyne has urged Unite members to back him in the leadership contest and stop the "hard left" in their tracks.

He said: "In seven days we will have the chance to prevent the hard left from getting the biggest cash bonanza in its sorry history.

"Jon Lansman tells us that if Len McCluskey is re-elected General Secretary of Unite, he is preparing to bankroll Momentum, the fringe group dedicated to taking over the Labour Party from within.

"Unite has already lavished £225,000 of its members' money on helping Jeremy Corbyn gain and retain the Labour leadership, as well as putting up £417,300 so that McCluskey can live rent-free and interest-free in a luxury London flat. Members of Unite have had enough of McCluskey treating their union as his personal piggybank.

"The hard left dream of seizing control of the Labour Party has a long history. It was tried by the Militant Tendency, who took over Liverpool Council in the 1980s."

He added: "This is fantasy politics. We have to stop McCluskey squandering Unite's money on it when our members are facing zero hours contracts, Brexit uncertainty and deepening job insecurity.

"Enough is enough. No more funding far left factions, no more luxury flats bought interest free and rent free. It's time for a change."