Labour leadership rivals Ed and David Miliband deny they are at war as the contest enters a critical phase.
David said it was "nonsense" he was personally criticising brother Ed by warning against Labour retreating into a left-wing "comfort zone".
Ed responded by warning about the party entering a "New Labour comfort zone" and ignoring its working class support.
But he insisted any differences between them were about the future direction of the party and not personal.
The contest - in which Ed Balls, Andy Burnham and Diane Abbott are also standing - is reaching its final stages with Labour party and trade union members receiving ballot papers next week.
The two Miliband brothers have emerged as the frontrunners, with both articulating their competing visions of how Labour should rebuild itself and regain power at the next election.
'Party of protest'
In a speech on Wednesday - billed as the most important of his campaign - David Miliband said Labour must not "look inwards or backwards" or it risked being out of power for generation as happened after its election defeats in 1951 and 1979.
Setting out ideas to hand more powers to communities, tackle social inequalities and build a more "moral economy", he argued that Labour needed both working class and middle class support if it is to "shift the centre ground of British politics".
In an earlier article in the Times, Mr Miliband - who won the backing of influential Labour backbencher Jon Cruddas on Wednesday - suggested that Labour risked falling back into a "comfort zone" of opposition.
This could see it marginalised as a "party of protest, big in heart but essentially naive, well meaning but behind the times", he wrote in the newspaper.
This has been interpreted as veiled criticism of his brother's leadership campaign in which the shadow climate change secretary has focused more on appealing to traditional Labour voters.
Ed Miliband - who has himself been endorsed by the New Statesman magazine - hit back by warning that the party risked getting stuck in a "New Labour comfort zone".
He said it had to avoid thinking "we got it right in the 1990s and we just need to keep on the same tack".
"That is a recipe for us not winning the next election. We have got to have the courage to change the party," he told the BBC.
It was "insulting" to suggest he was relying on a "core vote" strategy, arguing that Labour could not win an election without working class support and that, to do so, it needed to rethink its policies on issues such as tuition fees, low pay and civil liberties.
But as the rhetoric of the contest intensified, both brothers denied they had gone back on the commitment as the start of the campaign to refrain from making personal attacks on each other.
While acknowledging there was a "honest disagreement about the kind of change that Labour needs to make", Ed Miliband said relations between the two of them remained "very good".
Asked about the comments in the Times article, Mr Miliband said his brother "must be talking about someone else" but he stressed the candidates "must take special care to continue our debate in the spirit we started out".
And David Miliband said the contest remained "comradely and fraternal" - in contrast to the "civil wars and psychodramas" of previous Labour leadership battles - and that all the candidates were on the "same side".
The three other candidates have also been stepping up campaigning as the start of voting nears.
Shadow education secretary Ed Balls, who met postal workers on Wednesday, accused the government of a "shocking assault" on the welfare state after a report suggested low income families with children will be hit hardest by June's Budget.
Former health secretary Andy Burnham - who launched his policy manifesto on Monday - is continuing to make the case for further tax rises as an alternative to deep spending cuts as a way of cutting the deficit.
Backbench MP Diane Abbott has criticised what she says is the similar background of her four male rivals, describing them as "geeky young men in suits", and said she best embodies the values of grass roots Labour supporters.
The result of the contest will be announced at the start of the annual party conference, on 25 September.