The Momentum steering group is urging its members to back Rebecca Long-Bailey to succeed him.
Following a meeting of the steering group of the movement that campaigned behind Jeremy Corbyn, it said it had agreed unanimously that the shadow business secretary is the “only viable candidate” to continue Mr Corbyn’s “socialist agenda”.
The group said it is also recommending its members support shadow education secretary Angela Rayner as deputy, arguing the two would work well together to form a “united front against the Tories”.
It will now ask its members whether they agree with the recommendations, with ballots – consisting of just two questions – to be sent out early next week.
The group’s support for Ms Long-Bailey is unsurprising given that she has long been the favoured candidate of the left to take on Mr Corbyn’s mantle.
However many Corbyn supporting MPs are backing shadow justice secretary Richard Burgon for deputy rather than Ms Rayner, and Momentum’s support will be a significant boost for her campaign.
Angela Rayner associated with undermining Jeremy Corbyn
Although she already has the support of her close friend Ms Long-Bailey, Ms Rayner is distrusted by some on the left after backing Andy Burnham for leader in 2015 rather than Mr Corbyn.
Reports have suggested some around the Labour leader believe she was responsible for trying to undermine him.
A Momentum spokesman said: “We need a new generation of left wing MPs to lead our party and build on Labour’s popular policy agenda.
“Our co-ordinating group believe Rebecca Long-Bailey is the only viable candidate who will build on Labour’s vision for the future, deepen democracy in the party and unite all of our heartlands at the next election.
“We also need a leadership team who trust each other, work well together and will lead a united front against the Tories, and that’s why we’ve… recommended support for Angela Rayner.
“While it’s important left candidates get the support they need, this is a huge decision and that’s why we’re balloting our members.”
However there was some anger at the way the steering group appeared to have pre-empted the outcome of the membership vote.
Momentum criticised for decision before balloting its members
Laura Parker, a leading figure in the movement, tweeted: “Although I am pleased Momentum’s governing body accepted the principle of balloting its members on the leadership, I’m sorry they seem to have decided in advance what the answer is.
“Members should be able to choose from all leader and deputy candidates.”
Meanwhile, Ms Long-Bailey appeared to take a swipe at the leadership frontrunner – shadow Brexit secretary Sir Keir Starmer – saying the party’s backing for a second EU referendum had cost it votes in the last month’s general election.
She said that unlike Sir Keir – who has a north London constituency – she represented Leave voting Salford and Eccles, and understood the anger felt by many outside the capital.
“I’m not from London, and I’ve grown up seeing and being quite angry about the shift of power and wealth to the centre of London and Westminster,” she said.
“That’s what a lot of our communities are very, very angry about so it’s important to have a politician that doesn’t represent the political establishment now and the Labour Party does need to carefully consider, which of the candidates would best do that.”
Keir Starmer, favourite among Labour MPs, says party must unite
Meanwhile Sir Keir, launching his leadership campaign in Manchester, warned the party had “a mountain to climb” if it was to regain power after four straight general election defeats.
He said they had to come together and “unite as a party and as a movement” if they were to stand a chance of ousting the Conservatives.
“We cannot fight the Tories if we are fighting each other. Factionalism has to got to go,” he said.
Sir Keir remains the clear favourite among Labour MPs and was the first of the six contenders for leader to secure the 22 nominations required to progress to the next stage of the contest.
Ms Long-Bailey and the backbenchers Lisa Nandy and Jess Phillips have also passed the threshold.
However shadow foreign secretary Emily Thornberry, with 10 nominations, and Clive Lewis, with just four, appear to be struggling.
They have until 2.30pm on Monday to get the necessary support or be forced to drop out.
By Ben Gelblum, Gavin Cordon and Josh Payne
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