Labour leadership candidate Rebecca Long-Bailey has secured her place on the final ballot, having won the endorsement of the Fire Brigades Union (FBU).
The shadow business secretary joins the current frontrunner Keir Starmer and backbencher Lisa Nandy in the next round.
To make it onto the ballot put to party members, candidates have to win the support of three Labour affiliates - including at least two trade unions.
Long-Bailey already had the support of the Unite union and the bakers’ union BFAWU.
Alternatively candidates can proceed by securing the nomination of 33 constituency Labour parties (CLPs).
This is the route Emily Thornberry, the shadow foreign secretary, appears to need to take - but so far looks to be struggling.
In a statement on Wednesday, the FBU said: “There can be no going back to the so-called centrism of New Labour – we never again want to see our party turn its back on workers, public ownership, and its own members.
“We will be actively campaigning for Rebecca. Only she has what it takes to transform the country, win back Labour’s heartlands, and put power back in the hands of working-class people.”
The FBU endorsed Richard Burgon, the shadow justice secretary and a favourite of the left of the party, in the race to be deputy party leader. The decision means he has made it onto the ballot of party members.
The current frontrunner in the deputy race is Angela Rayner, the shadow education secretary.
Jeremy Corbyn’s successor as leader will be announced on April 4.