Britain's biggest trade union Unite is backing veteran left-winger Jeremy Corbyn as Labour leader.
The union's executive committee voted to lend its support to Mr Corbyn, with Andy Burnham as its second preference.
The union, which is Labour's biggest financial backer, supported Ed Miliband in the 2010 Labour leadership contest.
The unions will have less influence on this year's contest because the party has moved to a "one member one vote" system for choosing its leader.
But the decision is a big boost for Mr Corbyn, who has been well received at hustings involving trade unions in recent weeks.
Mr Corbyn has been taking part in a leadership hustings in Wales with his three rivals for the job - Andy Burnham, Yvette Cooper and Liz Kendall, all prominent shadow ministers.
'Force for good'
The Islington North MP said: "It is a great honour to receive Unite's nomination, and it underlines that this a serious campaign that has growing momentum.
"The leadership election is about one issue above others: whether we accept another five years of a race to the bottom based on cuts that destroy services and damage living standards, or whether we invest our way to a growth and fairness."
Who is Jeremy Corbyn?
First elected to Parliament in 1983, the former trade union official has campaigned on a succession of left-wing causes, including the Campaign for Nuclear Disarmament (CND) and the Stop The War coalition, and is a columnist for the Morning Star.
He has frequently been at odds with his party, opposing the Iraq war and other foreign interventions and backing public ownership of the banks. Voting records suggest he has rebelled against Labour 533 times since the party came to power under Tony Blair in 1997.
He has said his leadership campaign marked the launch of a broader anti-austerity movement.
He also thanked other trade unions who have nominated him - Aslef and the BFAWU - and for the endorsement of two unions not affiliated to Labour, the FBU and the RMT.
He added: "Trade unions are a force for good, a force for prosperity and we should listen to them more. For Labour to win again it must show it is on the side of the majority."
Unite said its backing for Mr Corbyn was in recognition that his policies were most closely aligned with those of the union.
'Tories for Corbyn'
Backbench MP Mr Corbyn only made it on to the ballot for the leadership contest thanks to a last minute rush of support from Labour MPs who said they wanted to widen the debate about the party's future, but would not be voting for him as leader.
He told BBC Radio 5's Pienaar's Politics he was "very grateful" to those MPs who had backed him.
"People want to have a debate about Labour's direction. I didn't want a leadership contest. I wanted a policy debate in the party. The National Executive decided that we'd have a leadership contest and so this is a very important way in which we can discuss the direction of the party," said Mr Corbyn.
And he hit back at the "Tories for Corbyn" campaign, which has reportedly seen supporters of the government signing up as registered Labour supporters for £3, to vote for him because they believe his election would boost the Conservative cause.
"If people do not support the Labour Party and do not wish to vote Labour, either in future elections or have done so in the past, they should not be registering as Labour supporters, it's not an honest thing to do," said the MP.
Unite said it would not be nominating anyone for deputy Labour leader but would be making a recommendation to members to support Tom Watson and Angela Eagle.
The decisions on who to support were taken on Sunday following debate by the union's executive committee, an elected body of 63 men and women from workplaces across the UK.