Unite, the UK's largest trade union, has backed left-wing firebrand Jeremy Corbyn for the Labour leadership.
The union decided to support Corbyn at a private executive board meeting today, and will advise members to vote for Andy Burnham as their second preference.
No official nomination was made for deputy leader, but the Union gave their support to Tom Watson and Angela Eagle for the role.
Mohammad Taj, the former president of the Trades Union Congress, confirmed the news on his Twitter account.
Corbyn, seen as an outsider in the the leadership race, barely scraped onto the ballot - only gaining enough nominations from MPs in the final minutes before the deadline.
In a statement, the Union said: "The backing for Jeremy Corbyn was in recognition that his policies were most closely aligned with those of Unite.
"Further, Unite confirmed that there would be no nomination made for the position of deputy, but a recommendation will be made to members that they support Tom Watson and Angela Eagle to reflect the strong support for these candidates, by using their first and second preferences as they see fit.
"The decisions on who to support were taken today following debate by the union’s executive committee, an elected body of 63 men and women from workplaces across the UK."
Unite overwhelmingly backed Ed Miliband in the 2010 leadership race, and the union's block vote helped him narrowly beat his brother, David.
Mr Corbyn 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.
"I also want to thank other unions who have nominated me - Aslef and the BFAWU - and for the endorsement of two unions not affiliated to Labour, the FBU and the RMT. Trade union members are the men and women whose money worries and anxiety for the prospects of their children, are now the every day reality of these Tory austerity years. The whole of trades union history has been based on protecting people from austerity.
"Without trade unions there would be no equal pay-act, no minimum pay, no Labour Party.
"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.
"I thank Unite for their nomination, not just for myself but for all those people who have already brought their time, efforts and hope to my campaign."
Last week, Corbyn submitted a motion to the House of Commons banning workers from having to work in temperatures higher than 30C.
Corbyn is still a 16/1 outsider for the Labour leadership, trailing behind frontrunner Andy Burnham, shadow home secretary Yvette Cooper and shadow care minister Liz Kendall.
His radical approach could see Labour vowing to scrap Trident nuclear weapons and introducing a £10-an-hour minimum wage.
He came last out of the four leadership candidates in a survey asking who would be best for the Labour party in the 2020 elections- but his popularity on Twitter comes out top of all those in the running.
A survey done by the Independent of 2,000 people showed that 18% thought Corbyn would enhance the chances of the Labour Party in the next election and 33% thought he would do the most harm to the Labour party.