Labour’s deputy leader will plead with Jeremy Corbyn to “deal with” Momentum in a showdown meeting today – warning the party could die if he fails to act.
Tom Watson will demand that Mr Corbyn immediately stamp on a hard-left plot by the group to seize permanent control of Labour, by joining forces with the super-union Unite.
The plot appeared to have the “tacit approval of the leadership”, Mr Watson said – but argued, of Mr Corbyn, that it was unclear “what he knows and doesn’t know”.
Vowing to demand answers today, he said: “I regard this is a battle for the future existence of the Labour party.
“This is high stakes. I hope my fellow members are going to understand that - and our leadership. I hope he is going to deal with Jon Lansman [the Momentum founder].”
The crunch meeting follows the weekend leaking of a secret recording of Mr Lansman addressing supporters at a meeting in Richmond, South London, on March 1.
On the tape, obtained by The Observer, the Momentum founder said Unite would formerly affiliate with Momentum, if his ally Len McCluskey secures re-election as its general secretary next month.
The plan appeared to be for the country's biggest union - and Labour's largest donor – to channel its huge funds, as well as organisational support, to the grassroots group.
Mr Lansman also called for a “a fair election” when Mr Corbyn steps down, backing a cut in the current requirement for leadership candidates to have the backing of at least 15 per cent of MPs and MEPs.
And he appeared to dismiss pledges made to Mr Corbyn to exclude supporters of the Socialist Party - the successor to Militant Tendency - from Momentum, saying: “No one from the centre is going to tell you to kick people out.”
Speaking on BBC Radio 4’s Today programme, Mr Watson said: “What Jon Lansman has outlined is a plan with Len McCluskey, the leader of Unite, to take control of the Labour Party.
“We have never seen the biggest union organise a political faction within the Labour Party with the tacit approval of the leadership.”
On Mr Corbyn, Mr Watson added: “I’m not sure if Jeremy knows there’s a secret plan. I’ll be raising it with him today.”
Last night, the deputy leader took on Mr Lansman on Twitter, saying: "You've revealed your plan. If you succeed you will destroy the Labour Party as an electoral force. So you have to be stopped."
But, in further evidence of Labour’s bitter internal wars, Christine Shawcroft, a director of Momentum, accused the party’s “hard right” of trying to block the democratic right of its members to shift its direction.
She argued Mr Corbyn is a “moderate socialist”, adding: “Jeremy’s policies are just pure common sense. I happen to think that Mr Watson is rather right-wing
“He is clearly wishes to go back to command-and-control on a Blairite model and that is not what the Labour Party is about. We have a mass membership now and that membership wants its voice to be heard.”
Ms Shawcroft dismissed the row over the secret recording as a “non story”, insisting: “John said nothing that was at all controversial.”
But Mr Watson said the tape showed Mr Lansman saying his plan was more important than winning a snap general election, adding: “That shows what his priorities are.”
He urged Labour members to fight any attempt to lower the threshold for leadership candidates to get on the ballot, when the party’s conference votes on the controversy this autumn.
Mr Corbyn was able to secure the required 15 per cent backing, but only after non-supporters nominated him to “widen debate” – which is unlikely to happen again, with a future left-wing candidate.
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