Jeremy Corbyn has received another boost in his bid to become Labour leader after receiving the backing of one of the country's biggest trade unions.
Unison leaders decided to nominate the London MP, with Yvette Cooper as its second choice.
The union's declaration was made after Ms Cooper and fellow leadership hopeful Andy Burnham warned party members they risk consigning the party to years in opposition if they choose left-winger Mr Corbyn as successor to Ed Miliband.
The warnings came after reports of a private poll giving Mr Corbyn a lead of more than 20 points over Ms Cooper on first preference.
But Mr Burnham's campaign sought to counter the figures by releasing their own polling data, which suggested their candidate holds a lead of almost 10 points over Mr Corbyn.
The shadow health secretary's camp said 29,020 people had been polled since July 15, with 10,828 - 37.31% - making Mr Burnham their first preference compared to 8,008 votes - 27.59% - for Mr Corbyn.
Almost 20% of people polled indicated they have yet to decide who they will vote for while Ms Cooper was in third place with 2,785 votes (9.6%) and Liz Kendall in fourth with 1,675 votes (5.77%).
Meanwhile, leaders of the GMB decided to make no recommendation on who their members should vote for.
Explaining Unison's decision, general secretary Dave Prentis said Mr Corbyn's message has "resonated" with public sector workers following pay freezes and redundancies.
Mr Prentis, who acknowledged members are free to vote for who they want, added: "They have been penalised for too long by a Government that keeps on taking more and more from them."
Unison represents 1.3 million workers, with 28,000 members in the Labour Party, and around 15,000 registered to vote in the leadership contest.
The union's decision not to pick Mr Burnham as either of its preferences has been viewed by some as a surprise move given the union represents many health sector workers.
Unison also decided to nominate Angela Eagle for deputy leader with Stella Creasy as its second choice.
Earlier, Ms Cooper informed party members it was time to "get serious", telling The Independent Labour would be "condemning our world to a Tory future" if Mr Corbyn wins.
And Mr Burnham said the party faces a "big choice" over "whether it is going to get straight back on a path towards government and getting a Labour government back or whether we are going to be a party of protest".
But Mr Corbyn, in an interview with the New Statesman, reiterated he is "in it for the long run" should he secure the leadership.
He added: "I hope the party would want to hold together and I'm sure it would.
"I hope the party would recognise that the most democratic election we have held has produced an important result and has mobilised more importantly a very large number of people.
"I've never seen so many people at Labour Party meetings."
Speaking in Rome, shadow business secretary Chuka Umunna said Labour should take its inspiration from the Partito Democratico of Italy's centrist Prime Minister Matteo Renzi, rather than Greek PM Alexis Tsipras's far-left Syriza, which has been praised by Mr Corbyn.
New odds from bookmakers William Hill, make Mr Corbyn favourite to win the contest for the first time, at 11/8, after posting him as 200/1 rank outsider when he first entered the race.
Voting opens on August 14, with the result due on September 12.