Rebecca Long-Bailey has won the backing of the Unite trade union in her bid to succeed Jeremy Corbyn as Labour leader.
The shadow business secretary now needs just one more union or affiliate group 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” to take on PM Boris Johnson.
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 make a “substantial” donation towards Mrs Long-Bailey’s campaign.
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.
To make the ballot, hopefuls need the support of three unions and affiliate groups representing 5% of the membership, or 33 local branches.
Brexit secretary Sir Keir Starmer and Wigan MP Lisa Nandy have already secured this level of support ahead of the 14 February deadline.
Having already been nominated by bakers’ union BFAWU, Unite’s support for Ms Long-Bailey means she needs just one more union or affiliate to join them.
However the fourth leadership contender, shadow foreign secretary Emily Thornberry, is yet to receive any union or affiliate backing and has only secured endorsements from three local branches so far.
Leader hustings cancelled
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 new leader and deputy will then be announced on 4 April.