The country’s biggest trade union chooses the left-winger over Andy Burnham. Mr Corbyn is also being backed by Conservative supporters who say he will make Labour unelectable.
Unite, which affiliates 500,000 of its 1.4 million members to Labour and gives the party £1.5 million a year, said it was backing Mr Corbyn as he is the candidate whose policies are the most closely aligned to the union.
The decision follows multi-union hustings held on June 30, a consultation with members’ elected representatives across the union, and a debate by the union’s executive committee, an elected body of 63 men and women.
United is the first of the biggest three unions to decide which of the four Labour candidates to back, with Unison and the GMB weeks away from making a decision. The remaining candidates in the contest are Mr Corbyn, Mr Burnham, Yvette Cooper and Liz Kendall.
Other smaller unions including the RMT have already come out for Mr Corbyn.
He 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 everyday reality of these Tory austerity years. The whole of trades union history has been based on protecting people from austerity.
Trade unions are a force for good, a force for prosperity and we should listen to them more. Jeremy Corbyn
“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 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 by using their first and second preferences as they see fit.
So to demonstrate our desire never to win again, Islington’s Jeremy Corbyn is now a Labour leadership candidate.
— John Mann (@JohnMannMP) June 15, 2015
Mr Corbyn won a place in the race thanks to a last-minute rush of support from MPs before the nomination deadline closed, with many admitting they were only backing him to widen the debate.
His nomination has infuriated some Labour MPs, with backbencher John Mann said Mr Corbyn’s presence in the contest demonstrated Labour’s “desire never to win again”.
Mr Corbyn is also receiving support from some Conservative supporters via a campaign called “Tories for Corbyn”, which is urging people to pay the £3 needed to get a vote in the leadership contest by becoming a registered Labour supporter.
The Telegraph columnist Toby Young said he had registered as a Labour supporter by filling in an online form and and paying the £3 fee.
He answered the question “Why did you sign up as a registered supporter?” with “to consign Labour to electoral oblivion”.
Mr Corbyn responded by saying: “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.”