Gordon Brown is expected to warn of the dangers of backing Jeremy Corbyn for Labour leader this weekend amid growing fears among senior figures that victory for the veteran left-winger could split the party.

Sources close to the shadow home secretary Yvette Cooper played down suggestions that the last Labour Prime Minister would officially endorse her campaign.

But Ms Cooper admitted she has spoken to Mr Brown about her leadership bid.

Opponents of Mr Corbyn hope Mr Brown’s intervention will be more influential than that of his predecessor as Prime Minister, Tony Blair.

Pollsters YouGov yesterday suggested that support for Mr Corbyn had risen after attacks from a number of Labour 'big beasts', including Mr Blair.

Mr Brown is due enter the debate with a speech on the future of the Labour Party at the Southbank Centre in London on Sunday, titled ‘Power for a Purpose’.

His decision to speak out follows growing concern within the Labour hierarchy that the party is heading for a decade of opposition, if not longer.

Yesterday, another of Mr Corbyn’s opponents Liz Kendall warned that the election of the Islington MP would be Labour’s resignation letter to the public.

The fourth MP in the contest, Andy Burnham, meanwhile, voiced hopes a silent majority of voters could defeat Mr Corbyn supporters.

Mr Burnham's team pointed to internal numbers which they suggested showed that the result could be tighter than originally thought, with Mr Burnham currently in second place.

Ms Cooper’s team, however, insists she is best placed to beat Mr Corbyn.

One major unknown for all teams, including Mr Corbyn's, is the potential impact of 'second' votes.

If no one candidate reaches 50 per cent in the first round of counting, the second preferences of the MP who came fourth will then be shared out.

The process continues until a winner is eventually declared.

Calls for two of the candidates to drop out, to create a straight run off between Mr Corbyn and an "anyone but Jeremy" candidate have been rejected.

Today the popular former Home Secretary Alan Johnson will use a speech in Hull to warn the party could face years in the political wilderness.

He will back Ms Cooper’s attack on Mr Corbyn earlier this week, in which she accused the MP of setting out policies that were not radical and would condemn Labour to opposition.

Mr Johnson will claim Labour faces a “clear choice” – “a weak, divided party in opposition for a generation or a Party that picks itself up, takes on the Tories, and defends the great things Labour did in government, but sets out a new path for the future.”

Despite the findings of pollsters, there was a slight blow to the Corbyn campaign yesterday when a former Welsh First Minister disassociated himself from its praise.

Rhodri Morgan said that Mr Corbyn's "hard left" campaign had "nothing to do" with his own "clear red water" policies for Wales.

Mr Morgan also backed Ms Cooper saying she had the "strength" to be Labour leader.

He said: “There's a massive gap between what you could call Corbynism and Blairism, and I attempted to fill that gap with what I call classic Labour."

"He's (Corbyn) not classic Labour - he's old, hard left Labour, so it's got nothing to do with my clear red water speech at all."

He added that he was “sure” Mr Corbyn did not envisage himself as a Prime Minister.

It has also emerged that Mr. Corbyn’s own brother Piers, who stood against Labour in May’s General Election, has been barred from voting for him.

Labour has been attempting to weed out so-called “'infiltrators”, including those from the hard left, who have signed up to take part in the leadership contest.

Yesterday party officials began the process of sending out more than 600,000 ballot papers.

The winner will be announced on September 12..