Ed Miliband has been accused of being "tetchy and menacing" as his row with the Daily Mail over his father continues.
The newspaper is refusing to apologise after publishing a headline on Saturday claiming Ralph Miliband "hated Britain".
Saturday's article argued the socialist professor's sympathy with communist ideology meant he was an enemy of the British state.
And it claimed 'Red Ed' Miliband's own policies as Labour leader, including the proposal to freeze energy prices unveiled last week in Brighton, are motivated by comparable 'socialist' motives.
"He was a man with a great sense of humour so the idea of me being part of some 'sinister' Marxist plot would have amused him and disappointed him in equal measure," Ed Miliband wrote in a 'right to reply' piece for today's Mail newspaper.
"He would have known it was ludicrously untrue. I want to make capitalism work for working people, not destroy it."
David Cameron has refused to condemn the Mail for its attack on the Milibands, claiming he had not read either of the articles.
But he said he would want to defend his father's reputation if someone "had a go".
"What's required is judgement - from politicians about what to say, from papers about what to print," the prime minister told the Today programme this morning.
"It's a woolly answer but it's an ongoing debate."
An editorial from the Mail defended the original article, saying the focus on Ralph Miliband was justified after Gordon Brown's former spin doctor Damian McBride referred to Ed's "attempt to achieve his father's vision" in his memoirs.
"With this testimony before us, from a former Labour spin doctor who knew Mr Miliband inside out, the Mail felt a duty to lay before our readers the father’s vision that is said to have inspired our would-be next prime minister," it stated.
"Yes, as his son argues, Mr Miliband Snr may have felt gratitude for the security, freedom and comfort he enjoyed in Britain.
"But what is blindingly clear from everything he wrote throughout his life is that he had nothing but hatred for the values, traditions and institutions - including our great schools, the Church, the Army and even the Sunday papers - that made Britain the safe and free nation in which he and his family flourished."
The Mail fears the privy council, of which Miliband is a member, will push forward with its attempts to impose a regulatory body on the national press when it meets next week.
Labour has played a key role in calling for a watchdog with teeth and Miliband showed no signs of relenting after the latest attack on his family.
"The Daily Mail sometimes claims it stands for the best of British values of decency. But something has really gone wrong when it attacks the family of a politician - any politician - in this way," he added.
"It would be true of an attack on the father of David Cameron, Nick Clegg, or mine.
"There was a time when politicians stayed silent if this kind of thing happened, in the hope that it wouldn’t happen again. And fear that if they spoke out, it would make things worse. I will not do that. The stakes are too high for our country for politics to be conducted in this way. We owe it to Britain to have a debate which reflects the values of how we want the country run."