Ed Balls has denied being aware of the briefings being carried out by his ally, former spin doctor Damian McBride.
But the shadow chancellor's protestations were met with scepticism by Westminster journalists.
Speaking to the BBC on Monday morning, Balls said McBride had been "despicable" and denied that he had ever himself given negative briefings against colleagues while in government.
Asked whether he could categorically state that he was never involved in negative briefing against a government colleague, Balls said: "Yes. That's not something I've ever done, I think it's the wrong way to do politics.
"Damian McBride has come out and said he did some of those things. It was despicable. It's the wrong thing to do."
Balls was a key member of Gordon Brown's inner circle with McBride during the last Labour administration.
Ed Balls has just said: “I didn’t know Damian McBride was doing personal briefings against ministers.” No, really, he actually said it.— James Kirkup (@jameskirkup) September 23, 2013
Benedict Brogan, from the Daily Telegraph, said Balls' claims had been met by "a chorus of dropped marmalade and spluttered coffee."
In the latest revelations from his memoir Power Trip, published in the Daily Mail, McBride accused party leader Ed Miliband of turning against him in an effort to further his own career.
McBride claimed he was unfairly accused of blaming Miliband and Douglas Alexander - now shadow foreign secretary - for the decision not to call an election in 2007 at the height of Brown's popularity.
And he said Miliband found him a "convenient person to blame" as he tried to distance himself from the Brown circle.
Details of the toxic infighting within the Labour administration during the Blair-Brown years have cast a shadow over the Labour conference in Brighton and led a string of senior figures publicly to condemn McBride.
But Balls insisted that the party had changed under Miliband's leadership.
He said: "This kind of negative, nasty briefing is wrong. But I think also it's a thing of the past.
"The Blair/Brown era is gone. It is not how Ed Miliband and I are doing things in the Labour Party today. There's been none of it for the last three or four years. Thank goodness for that. We're in a better place now."
Asked if he had used McBride's services to undermine potential rivals in the race to succeed Brown as party leader, Balls said: "If I ever did, it didn't work very well, did it, because I didn't become the party leader.
"No, it's utter nonsense.
"There's always going to be people who will in an off-the-record smearing way make allegations, but it's not true and there's no evidence for that at all. It's not something I've ever done, it's not something I will ever do."