Leaders of the country's biggest trade union have voted to back Jeremy Corbyn for the Labour leadership.
The executive of Unite chose Corbyn ahead of Andy Burnham, who was second preference.
The decision is a big boost for Mr Corbyn, who has been well received at hustings involving trade unions in recent weeks.
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."
Unite said its backing for Jeremy Corbyn was in recognition that his policies were most closely aligned with those of the union.
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.
It follows the multi-union hustings held on June 30 and consultation with members' elected representatives across the union.
Unite is Labour's biggest union affiliate, with members across the private and public sectors. It affiliates 500,000 of its 1.4 million members to Labour, giving the party £1.5 million a year.
It is the first of the big three unions to decide which of the four Labour candidates to back, with Unison and the GMB some weeks away from making a recommendation.
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