The left wing grassroots Momentum group which propelled Jeremy Corbyn to the Labour leadership is urging its members to back Rebecca Long-Bailey to succeed him.
Following a meeting of its steering group, 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.
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.
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 & Deputy candidates. https://t.co/Y2gfUNJKS9
— Laura Parker (@ParkerCiccone) January 11, 2020
"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.
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."