The GMB union has backed Owen Smith to become Labour leader, after balloting its membership for their views.
Mr Smith was endorsed by 60% of the 43,419 union members who voted, while 40% supported incumbent Jeremy Corbyn.
The union is one of the biggest affiliated to the party, with about 641,000 members - and is the biggest to back Mr Smith to date.
GMB boss Tim Roache said he would "proudly campaign" alongside Mr Smith, who welcomed the union's support.
Mr Smith said it was "a great honour" to receive GMB's nomination.
"I look forward to working with the GMB, its members and all in the labour movement to tackle inequality, invest in strong public services, and for Britain to have trade union and workers' rights that are the envy of the world," he said.
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The GMB did not endorse any candidate in last year's Labour leadership election, but at a its annual GMB congress earlier this year the union called for "clear and unambiguous" support for Mr Corbyn.
The union also supported a motion which called for the de-selection of any MP who refused to put on record their backing for the Labour leader.
In a statement announcing the results of the consultative ballot, GMB general secretary Mr Roache said Labour "is at a crossroads" in "dangerous political times", and that it was "time for us to face up to reality".
"I'll proudly campaign alongside Owen in the coming weeks and months to deliver on his pledges to end the public sector pay freeze, to support defence workers, to invest in health and industry, and to build an industrial strategy - these are bread and butter issues for the people I represent.
"But we can only tackle them if we're in government. That's the end game and I hope whatever the final result, the Labour Party will unite when all this is over.
"GMB members cannot afford for Labour to be talking to itself in a bubble for the next five years while the Tories run riot through our rights at work, our public services and our communities."
The union represents workers on the UK's Trident nuclear weapons programme.