The Unite Executive Council voted to support Jeremy Corbyn in the Labour Leadership contest with Andy Burnham as second preference.
Unite joins ASLEF and BFAWU, two other Labour-affiliated unions, in backing Jeremy, as well as RMT and FBU who are not affiliated.
Jeremy 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 trade 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."