Former Labour leader Neil Kinnock has given his endorsement to Andy Burnham in the battle for the party's top job, declaring his belief that the shadow health secretary is capable of winning back power.
In a clear warning to supporters tempted to back Jeremy Corbyn, Lord Kinnock cautioned that "it won't be enough to protest ... our proposals for action must be able to attract votes from the breadth of the British people".
And he warned that some of Mr Corbyn's support may be coming from the "Trotskyite left and the Telegraph right" who are backing the veteran left-winger for "their own malign purposes" rather than out of Labour's best interests.
Mr Corbyn has issued an appeal for unity after Mr Burnham warned the party risks being split by "factionalism" and "provocative" comments. His warning came after Corbyn supporter Dave Ward of the Communication Workers Union described the Islington North MP as the antidote to the Blairite "virus".
In a statement, Mr Corbyn called for "comradeship and unity in action" during the remainder of the debate leading to the September 12 unveiling of the successor to Ed Miliband.
"The leadership election should be conducted with one thought in mind: our objective is to be a united party focused on winning the general election and campaigning across the country, day in day out," said Mr Corbyn, the surprise bookies' favourite who won more nominations from constituency parties than any of the other three contenders.
Dismissing predictions of schism, Mr Corbyn insisted: "When the dust has settled, we will still all be Labour."
Liz Kendall - seen as the candidate of the Blairite centre-left - accepted Mr Corbyn is ahead at this stage, but insisted: "Things will change."
Party members are "desperate for an alternative to where we have been for the last five years, and they want hope for the future," she told The Observer.
But she said Labour had no chance of election victory with the far-left backbencher at the helm. And she said: "Jeremy Corbyn does not have a monopoly on hope or a monopoly on setting out an alternative. For the remaining six weeks of the campaign, I am going to be setting out our credible alternative of hope for the future."
Ms Kendall is due on Monday to unveil plans to tackle inequality, expected to include £1 billion for pre-school support for children, paid for by reversing George Osborne's inheritance tax cuts.
Announcing his endorsement in an article for The Observer, Lord Kinnock said: "In the leadership election, we are not choosing the chair of a discussion group who can preside over two years or more of fascinating debate while the Tories play hell with cuts. We have to elect a leader capable of taking us to victory in the 2020 election and of being a Labour prime minister."
Acknowledging party fears that the £3 fee to become a registered supporter was encouraging "entryism" by individuals intending to vote for Mr Corbyn in the hope of causing problems for Labour, Lord Kinnock said: "The Trotskyite Left and the Telegraph Right who might participate in this election clearly have their own malign purposes."
But he said for sincere Labour supporters and the people they aim to help "it won't be enough to protest – our ideas and ideals must appeal and prevail".
Mr Burnham had "the radical values which come from his background, the experience as a campaigner and a Cabinet Minister" and was offering "credible policies" and producing "usable progressive answers to the problems which afflict our age", said Lord Kinnock.
"A Labour Party led by Andy will restore its strength, regain wide support and win," he predicted.
Mr Corbyn indicated he would seek to appoint a shadow cabinet representing all shades of opinion in the party if he was elected leader.
Asked if he would offer a place to Ms Kendall, who has said she would not serve under him, he told The Observer: "Yes. Of course there are differences of opinion and I have to be big enough to accommodate those differences of opinion and I understand that.
"Some colleagues have said they would not be very keen on working with me, but I am sure these things were said in the heat of the moment."
He dismissed suggestions that his support was being inflated by mischief-makers who were not true Labour backers.
"The idea that it is some kind of entryist plot is absolute nonsense," said Mr Corbyn. "It is people who are serious about their politics and want the Labour party to be able to represent them and they have felt rather unrepresented in the past years."
He insisted he was "cautious" about polls showing him in front and acknowledged it would be difficult to sustain the "high level of intense activity" which has seen him pack halls up and down the country with supporters, many of them young people enthused by his anti-austerity message.