Sir Keir Starmer has won the backing of the UK's largest trade union, Unison, to become the next Labour leader.
Unison, which has 1.3 million members, said the shadow Brexit secretary was best placed of the candidates to unite the party and regain public trust.
Unite, Labour's largest financial backer, will decide later this month who to back in the contest.
Sir Keir has also become the first to secure enough nominations from MPs and MEPs to get on the ballot paper.
The Holborn and St Pancras MP has, so far, secured the backing of 41 colleagues - well above the minimum of 22 required.
Shadow business secretary Rebecca Long Bailey is his closest competitor, followed by Jess Phillips.
Nominations close on 13 January and the winner will be announced on 4 April following a ballot of party members, trade union and other affiliates and registered supporters.
As the contest gathers pace, BBC Newsnight correspondent Lewis Goodall reported that Barry Gardiner, the party's international trade spokesman, could become a candidate. The Brent North MP has yet to confirm this.
Unison's endorsement, which was agreed by a committee of "working people from across the country", is a major boost for Sir Keir's campaign.
Announcing its decision, the union said it believed the former director of public prosecutions was capable of taking Labour back into government. General Secretary Dave Prentis said working people depended on Labour being in power to change their lives.
"We believe - if elected by the membership - Keir Starmer would be a leader to bring the party together and win back the trust of the thousands of voters who deserted Labour last month," he said.
"Keir has a clear vision to get Labour back to the winning ways of the past. He is best placed to take on Boris Johnson, hold his government to account and ensure Labour can return to power."
The union, which represents workers across the NHS, schools and other public services, backed Jeremy Corbyn in 2015 and 2016.
The BBC's political correspondent Iain Watson said Sir Keir was the overwhelming choice of the union's Link Committee, having won more support than all of the other candidates combined.
He has also recruited Simon Fletcher to his campaign team - Jeremy Corbyn's former chief of staff who ran his successful leadership bid in 2015.
Mrs Long Bailey has Momentum founder Jon Lansman as her campaign director and former spokesman for Mr Corbyn, Matt Zarb-Cousin, as her director of communications.
Sir Keir told the BBC he was "delighted" by the endorsement but played down talk of him now being the frontrunner, telling the BBC "there was a long way to go" in the contest.
"What I want to do is lead a united Labour Party that works with trade unions to bring them together to face the future," he said.
It caps a good day for the shadow Brexit secretary, whose growing number of parliamentary backers include former foreign secretary Margaret Beckett, the new shadow culture secretary Tracy Brabin and Tottenham MP David Lammy.
Mrs Long Bailey's 17 backers include shadow home secretary Diane Abbott, while Birmingham Yardley MP Jess Phillips has the support of 16 colleagues, including Wes Streeting and Margaret Hodge - outspoken critics of Mr Corbyn over his handling of anti-Semitism allegations.
Wigan MP Lisa Nandy has 11 nominations, while shadow foreign secretary Emily Thornberry has three. Shadow Treasury minister Clive Lewis has secured his first nomination from Rachael Maskell.
Earlier, Mr Lewis rated Mr Corbyn "six out of 10" as leader of the Labour Party. Speaking to the BBC's Victoria Derbyshire programme, he criticised the outgoing leader for making mistakes on Brexit and dealing with anti-Semitism.
"But some things he got right," Mr Lewis added, "so in many ways he's renewed our party." The comments follow those of Mrs Long Bailey, who rated Mr Corbyn 10 out of 10 for his performance as leader, despite Labour's electoral defeat.
In the race to become Labour's next deputy leader, shadow education secretary Ms Rayner way out in front so far in terms of nominations with 45 and has also won Unison's backing.
Before entering Parliament, Ms Rayner was a local Unison representative while employed as a care worker in Greater Manchester.
Ian Murray, Scotland's only remaining Labour MP, is second in the list of declared backers so far while shadow justice secretary Richard Burgon is third.
As of Wednesday evening, shadow women and equalities minister Dawn Butler and Tooting MP Rosena Allin-Khan both had seven nominations. The remaining candidate, Birmingham Perry Barr MP Khalid Mahmood, had two.