13/08/2015 15:29 BST | Updated 13/08/2016 06:12 BST

Andy Burnham Issues Appeal As Yvette Cooper Criticises Jeremy Corbyn

Jeremy Corbyn's closest rivals for the Labour leadership have clashed over how to defeat him - Andy Burnham urging a "positive campaign" while Yvette Cooper laid into the left-wing favourite.

As the party prepared to send ballot papers to the more than 600,000 who will have a vote in the contest, candidates continued to raise concerns about the fairness of the rules.

Liz Kendall conceded she had "a hell of a long way to go" to convince enough people to back her as she prepared to embark on a whistlestop tour to every part of Britain in a bid to revive her flagging campaign.

An increasingly heated contest was fuelled by former prime minister Tony Blair's stark warning that electing Mr Corbyn, who continued to pack out halls as he took his bandwagon to Scotland, would result in "annihilation" for the party.

Mr Burnham - the closest challenger according to polls suggesting Mr Corbyn is on course for a shock victory - said it was unhelpful to "second guess" the outcome and make such "dire predictions".

"I will be keeping it positive," he told the Press Association.

"I think people are well aware of the issues at stake and I think the time has come to trust the members of our party and the supporters of our party to make the right decision about its future," he said - later suggesting attacks on Mr Corbyn "misread the mood" of the party.

But Ms Cooper used a speech to criticise the frontrunner, accusing him of proposing "old solutions to old problems" and presenting herself as the "real radical".

She dismissed Mr Corbyn's plan to effectively print money to fund public investment as "really bad economics" and compared those who "bought into" Mr Corbyn's pitch to those whose yearning for "something subversive" had led them to switch to the SNP and Ukip.

"We have to look the 21st century in the eye, face up to the future. That's where we will find the new radicalism, the answers in the modern fight for social justice, equality and solidarity," she told a rally in Manchester.

"Not the old answers of the past. They won't change the world, they will keep us out of power and stop us changing the world."

Mr Burnham denied his campaign was "crying foul" over new leadership rules which have seen more than 120,000 people pay £3 to qualify for a vote - raising fears of political opponents undermining the process - and almost 190,000 from trade unions and other affiliates signed up.

He said it should be a "cause for celebration" that so many people had been attracted to take part so long as the party was able to weed out Tory MPs and others guilty of "despicable" attempts to subvert the result.

But he confirmed that he and rivals had raised formal concerns about their lack of access to the details of a late rush of new recruits.

Ms Kendall, who is trailing her three rivals after a strong start, told the Press Association: "I am a long shot in this campaign and I have a hell of a long way to go in making this case, but I will keep fighting in what I believe in to the very end."