Andy Burnham says only he can unite the Labour Party

Jeremy Corbyn's two main rivals for the Labour leadership are engaged in a bitter fight over who is best-placed to halt the left-winger's charge.

While the surprise frontrunner was addressing another packed public meeting, Yvette Cooper's campaign team called on Andy Burnham to "step back" in her favour.

They claimed polling showed she was now best placed to defeat Mr Corbyn - ahead of Mr Burnham in most parts of the country and attracting a significant majority of potentially-vital second preferences.

"If he isn't prepared to offer an alternative to Jeremy, he needs to step back and leave it to Yvette," a spokeswoman said.

"And he should do the right thing by the party and tell people who do still support him to put second preferences for Yvette - something he is still refusing to do."

But Mr Burnham's campaign chief hit back with a suggestion the shadow home secretary was refusing to give way "out of pride" despite facing a "hopeless" situation.

"The stakes are desperately high with the future electability of the Labour Party now in mortal danger," shadow transport secretary Michael Dugher told the Guardian.

"History will not be kind to those who appear to cling on out of pride when the situation really is quite hopeless. It's time now to rally behind the only person in this contest who can beat Jeremy Corbyn and that is very clearly Andy Burnham."

The dominance of Mr Corbyn - who left a hall in Ealing, West London, to address crowds of supporters unable to get in to hear him - has left his mainstream rivals scrabbling for position.

As the 600,000-strong electorate started to cast their votes, Mr Burnham sought to reach out to Corbyn supporters by offering to "involve" him in his team and saying there was a "good deal of common ground" between them.

It drew a similar commitment from the Corbyn campaign.

In contrast Ms Cooper has declared herself unwilling to work alongside the Islington North MP if he wins and has launched a series of attacks on his policy platform.

It came as the fourth candidate, Blairite Liz Kendall, was endorsed by former foreign secretary David Miliband.

The former MP - the latest in a string of party big beasts to speak out against Mr Corbyn - said the party needed "passionate reform not angry defiance".

Ms Kendall insisted neither of her rivals had asked her to drop out of the race to boost their chances of winning.

Mr Corbyn confirmed that he could work with Mr Burnham, if elected.

"This is an election which neither of us have been elected to yet. So it seems slightly forward to start offering positions," he told BBC2's Newsnight.

"Obviously there has to be a party of all the talents and of course we can work together.

Asked about the other candidates, he said: "I'm not sure everyone else is totally on that page as yet but I'm sure they will be in time."

The latest intervention by a senior party figure came from Lord Kinnock, who joined warnings that Labour risked becoming unelectable.

"I can see why people are angry and want to protest," the former Labour leader told the programme.

"But then they have got to make a decision whether they want to be part of a Labour movement which produced a political party to seriously contest democratic power or they want to be in perpetual demonstration, which is fulfilling and noble but ultimately rarely effective."

Shadow education secretary Tristram Hunt - a supporter of Ms Kendall who is giving his second preference vote to Ms Cooper - said he believed the contest remained "far more open than people think".

"The issue we have with Jeremy Corbyn is about the indulgence - people feeling good about themselves in the Labour Party rather than doing good for the country and that is what the Labour Party is about."