Rebecca Long-Bailey has won the backing of the Unite union in her bid to succeed Jeremy Corbyn as Labour leader.
The shadow business secretary now needs just one more union or affiliate to endorse her to confirm her place on the members’ ballot.
Unite’s general secretary, Len McCluskey, said Mrs Long-Bailey had the “brains and the brilliance” for the job.
The union will also back Richard Burgon for the vacant deputy leader post.
Speaking after a meeting in London, Mr McCluskey said his union would donate money to Mrs Long-Bailey’s campaign but an amount had not been decided.
The union, which was Labour’s biggest financial backer during last month’s election, had been widely expected to back her pitch for the top job.
After receiving the nomination, Mrs Long-Bailey said she was “honoured” to receive the union’s backing.
“I didn’t see myself as the kind of person who could ever become an MP. It was Unite, my trade union, that supported me to realise my potential,” she added.
Mr McCluskey said Unite’s executive committee had concluded Mrs Long-Bailey was “best placed to take the fight to the Tory party” on behalf of its members.
“She is standing for unity, socialism and the determination to make Johnson’s term in office short-lived,” he added.
He added that the union was confident that Mr Burgon would make a “superb deputy” for the party.
In an apparent swipe at former deputy leader Tom Watson, he said Mr Burgon would display “the qualities that have long been absent from that post,” including “pride in our values and “loyalty to their leader”.
Mr Watson was often at odds with the leadership during his time in the role, and faced an attempt to oust him at Labour’s conference last year.
Brexit secretary Sir Keir Starmer and Wigan MP Lisa Nandy have already confirmed their place for the members’ ballot stage.
The fourth leadership contender, shadow foreign secretary Emily Thornberry, is yet to receive any union or affiliate backing.
She has just three of the required 33 local branches required to secure her place in the next stage of the contest.
Sir Keir has cancelled all campaign events this weekend after his mother-in-law was involved in a serious accident. She remains critically ill in hospital.
He sent Labour’s Chris Matheson to the Unite meeting in his place.
Labour’s general secretary, Jennie Formby, also confirmed the hustings due to take place in Leeds between the leadership candidates on Saturday would be cancelled, although the deputy leadership event would go ahead.
She added: “We have sent our very best wishes and solidarity to Keir and his family, and our hope that his mother-in-law recovers very soon.”
Shadow education secretary Angela Rayner is the only candidate for the deputy leadership to have made it onto the ballot, with the support of Unison, the GMB, Usdaw and the NUM.
As well as Mr Burgon, the others in the running are Scotland’s only remaining Labour MP, Ian Murray, shadow equalities minister Dawn Butler, and Tooting MP Rosena Allin-Khan.
The deadline for all the candidates to get the support of either unions/affiliate groups or local parties is 14 February.
The new leader and deputy will then be announced on 4 April.