Ed Miliband said he has never been engaged in the sort of "reprehensible" briefings against Labour colleagues exposed in the memoirs of disgraced Gordon Brown spin doctor Damian McBride.
The Labour leader said he had urged Mr Brown to sack the controversial figure amid suspicions about his behaviour, which he said were "not my kind of politics".
And he insisted that he led a "unified" party and had laid down the law from the moment he became leader that such tactics would not be tolerated.
Mr McBride's account of vicious Labour in-fighting, briefing against rival ministers as members of the Blair and Brown camp tried to knife each other, has cast a shadow over the party's conference.
Ex-cabinet minister Dame Tessa Jowell said Mr Brown must publicly state that he did not know about the activities of Mr McBride or have his own reputation tarnished.
During an appearance on BBC1's Andrew Marr Show, Mr Miliband was asked whether he could "look into your heart during that period and say to yourself that you were completely clean".
"I think that people who know me would say that I am someone who has never engaged in the factionalism and was never engaged in the briefing," he said.
"That wasn't my style of politics. It has never been my style of politics and I find it reprehensible and not something I would engage in.
"I am someone who is deeply committed to the Labour Party and deeply committed to Britain and that is the way I have always approached my politics."
Questioned on whether he had turned a blind eye to the behaviour, he said: "Absolutely not. We can't have that. It diminishes politics in the eyes of the,public and it diminishes our democracy in the eyes of the public. I've got no tolerance for it."