Liberal Democrat leadership contender Chris Huhne has issued an apology over a paper describing leadership rival Nick Clegg as a "calamity".
Mr Huhne said he "sincerely apologised" for the title of the document and "disassociated himself from it".
Nick Clegg's campaign team have lodged an official complaint with the party's chief whip.
Senior party officials will meet on Monday to decide whether to take further action.
Mr Huhne, the party's environment spokesman, told BBC One's Politics Show he had no knowledge of the document.
But, in heated exchanges, he went on to accuse home affairs spokesman Mr Clegg of "flip-flopping" over his policies.
Mr Clegg said Mr Huhne was trying to create "synthetic differences" which "our opponents will use against us".
'Don't support it'
Three days ahead of ballot papers being sent to Lib Dem members, Politics Show presenter Jon Sopel produced a briefing document with the words "Calamity Clegg" at the top, which he said had come from the Huhne camp.
Mr Huhne said he had not seen this and said he apologised if it had been written by one of his aides.
He added: "I don't support describing anyone else in the party as a calamity."
Mr Huhne also said: "But I do think we've seen a series of flip-flops from Nick."
He said Mr Clegg had not established clear policies on nuclear weapons, and aspects of running schools and the health service.
Mr Clegg said: "I have said to you until I'm blue in the face that my position is very clear on all the issues... where what you are seeking to do is believe the worst that is said about me."
Mr Clegg also said: "As a colleague I will do you the respect of listening to what you've said."
During exchanges in which the two men frequently interrupted each other, Mr Huhne said of Mr Clegg's NHS policy: "You are flip-flopping on this."
He also said: "When I challenged you on this I didn't even get the courtesy of a reply from you. I got it from your campaign manager.
"Why are you trying to make sure there isn't a paper trail on these key issues ahead of the leadership if you're not attempting to face both ways?"
The acting leader of the Liberal Democrats, Vince Cable, told BBC 5Live the argument seemed typical of party leadership contests.
"From what I've heard so far it doesn't seem much worse than in most of the leadership contests that we've had in other parties as well as our own, so it's not a major departure I don't think," he said.
"But I'm sure they will take note of what's happened today and concentrate on policy issues and prepare for the battles that one or the other of them will have to face against our opponent."
A spokeswoman for Mr Huhne said the title of the leaked document "Calamity Clegg" had been thought up by an "over-zealous researcher".
A statement on Mr Huhne's website says: "There is no excuse for this. The document title had not been approved before the document was sent out.
"In no way does the title of the document as sent to the Politics Show represent Chris Huhne's opinion and he completely dissociates himself from it."
The spokeswoman added that Mr Huhne stood by the contents of the document - a list of quotations which she said showed Mr Clegg's policy "flip-flops".
The two men are the only contenders to replace Sir Menzies Campbell, who stepped down as Lib Dem leader last month after 19 months in the job.
A survey for BBC Radio 4's The World This Weekend in 94 constituencies held by the Lib Dems, or where the party has a good chance of winning at the next election, suggests support is stronger for Mr Clegg.
Some 43 chairmen backed him, while 30 opted for Mr Huhne. Another 12 said they were undecided and nine refused to say.