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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.

Meanwhile, Mr Corbyn has the backing of the UK's biggest union, Unite, among several others.

Unite has confirmed it won't be holding a members' ballot.

A Unite spokesman said: "Unite has formally nominated Jeremy Corbyn. It follows Unite's recent policy conference, where members representing all the union's industrial sectors and regions, along with the union's executive council, declared overwhelming support for Jeremy Corbyn in the leadership contest."

It's understood the second largest union, Unison, could endorse a leadership candidate on Thursday.

It is carrying out a consultation with members rather than a full ballot - and only those members who have opted in to a political fund within the union linked to Labour will be consulted. At the end of the process they could nominate a candidate or decide not to nominate at all.

Other unions supporting the Labour leader include the:

train drivers' union Aslef

Communication Workers Union

Fire Brigades Union

Transport Salaried Staffs' Association

construction workers' union UCATT

Mr Smith said it was "a great honour" to receive GMB's nomination - which comes on top of support from the Community Union, Musicians Union and shopworkers' union USDAW, the fourth largest.

He said: "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."

And he told the BBC he thought the nomination was "a real vote of endorsement of me, and it's a real shot in the arm for my campaign".

The GMB did not endorse any candidate in last year's Labour leadership election, but at its annual GMB congress earlier this year the union called for "clear and unambiguous" support for Mr Corbyn.

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".

GMB members, he said, "cannot afford for Labour to be talking to itself in a bubble for the next five years".

Mr Roache added: "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.

Image caption Mr Roache said the 60% to 40% result was "pretty outstanding"

Speaking to BBC News, Mr Roache said the question put to members had been "very carefully couched", and asked who they thought was the best candidate to serve as prime minister.

"Because that's what it's all about," he said.

"We can have the best shopping list, wish-lists for working people in the world, but without power in to 10 Downing Street it really does mean nothing."

The GMB union represents workers on the UK's Trident nuclear weapons programme.