Sir Keir Starmer has become the first Labour leadership candidate to get his name on the competition's final ballot, after another union came out in support of him.
The shadow Brexit secretary was nominated by retail trade union Usdaw, which also threw its weight behind deputy leadership candidate Angela Rayner.
Sir Keir says he is "honoured" to have received the endorsement, adding how he plans to "stand shoulder to shoulder with the trade union movement" if he becomes Labour leader.
Usdaw's general secretary Paddy Lillis said the union had backed the pair because they are "the right leadership team to unite and rebuild Labour after a devastating election loss".
Sir Keir won his place on the final ballot after adding Usdaw's endorsement to the support of Unison and the Socialist Environment and Resources Association (SERA).
The focus for Sir Keir will now turn to Labour Party members, affiliated supporters and registered supporters, who will eventually elect a successor to Jeremy Corbyn when voting opens from February 21 til April 2.
To progress to the competition's final stage, candidates must secure backing from three affiliates including two trade unions, or from at least 33 local Labour parties.
The deadline has now passed for people to join Labour to vote in the leadership contest, which will end with the announcement of the winner on April 4.
Shadow business secretary Rebecca Long-Bailey, shadow foreign secretary Emily Thornberry and backbenchers Lisa Nandy and Jess Phillips are still in the race to join Sir Keir on the ballot.
Ms Long-Bailey is another of the race's favourites, due to being regarded as Mr Corbyn's continuity candidate.
Ms Phillips and Ms Thornberry are yet to receive the backing of any union, however the latter has support from three local Labour parties and is expected to progress to the competition's final stage.