Owen Smith has appeared to call his Labour leadership rival Jeremy Corbyn a "lunatic".
Footage taken at a campaign event in Hammersmith, west London, showed Mr Smith telling an audience: "What you won’t get from me is some, you know, lunatic at the top of the Labour Party. You’ll have someone who tries to form a coherent narrative about what’s wrong with Britain.”
Mr Corbyn's campaign team told ITV News that Mr Smith should withdraw his remark and apologise to people suffering with mental illness.
But Mr Smith denied he was referring to Mr Corbyn but apologised to anybody he had offended with the remark.
"I didn't use that language with reference to Jeremy, I used it with reference to myself," he told ITV News. "If anybody's been offended by that, I apologise unreservedly."
It is not the first time the Labour leader contender has come under fire for comments he has made.
In July, Mr Smith was condemned for calling on his party to "smash" Prime Minister Theresa May "back on her heels".
He was also criticised for suggesting so-called Islamic State would have to be brought into peace talks to end the Syrian civil war.
A spokesperson for the Jeremy for Labour campaign said: "Owen Smith has degraded this contest by descending into personal abuse.
"He should apologise to people suffering with mental illness, many of whom would have been dismayed and upset to to hear such offensive language used in public by a Labour politician.
"He should also withdraw his remark, and spend time with people suffering from mental health problems to develop some sensitivity in his use of language.
"This is simply not the language that someone standing to lead our party should use, and it injects an ugly tone into this contest that no Labour member wants to see."