Labour says it is investigating allegations about the “comments and behaviour” of its MP Jared O’Mara.
Mr O’Mara has already apologised for homophobic and misogynistic remarks he made online in 2002 and 2004 and quit the Commons equalities committee.
But he denies subsequent allegations of offensive language to a constituent in March this year.
Labour said it was investigating the more recent claims, which were made on BBC Two’s Daily Politics.
“The party is investigating Jared O’Mara MP in relation to comments and behaviour which have been reported from earlier this year,” it said.
Sophie Evans told the Daily Politics that Mr O’Mara called her an “ugly bitch” in March 2017, three months before he was elected in Sheffield Hallam. Mr O’Mara said this was “categorically untrue”.
Rival parties have attempted to put pressure on the Labour leadership over Mr O’Mara, who unseated ex-Lib Dem leader Nick Clegg in June’s general election.
In a letter to Labour leader Jeremy Corbyn, Justine Greening, the Education Secretary and Equalities Minister, asked how Mr O’Mara had been selected as a candidate.
“Violent, sexist and homophobic language must have no place in our society, and parliamentarians of all parties have a duty to stamp out this sort of behaviour wherever we encounter it, and condemn it in the strongest possible terms,” she wrote.
“It is time you step forward, as leader of the Labour Party, and send a message that this sort of behaviour will not be tolerated.”
Lib Dem leader Sir Vince Cable called for Mr O’Mara to have the whip removed and urged Labour to review its general election selection process.
Mr O’Mara’s apology came after online posts from 2004 were published by the Guido Fawkes website. In them, he claimed singer Michelle McManus only won Pop Idol “because she was fat” and said it would be funny if jazz star Jamie Cullum was “sodomised with his own piano”.
More comments, involving homophobic language, then emerged dating back to 2002.
Mr O’Mara apologised to Labour MPs for these remarks at a meeting at Westminster on Monday evening.
His speech, which was described by sources as “emotional” and “heartfelt,” was met with applause by Labour MPs.
He then told online magazine Huck he had been “through a journey of education”.
He added: “I’ve stood down from the Women and Equalities select committee too – I think it’s the right thing to do. I don’t think I can continue on that committee when I feel so deeply ashamed of the man I was 15 years ago.”
Shadow chancellor John McDonnell said: “He has apologised for what we knew yesterday. He issued a profuse apology.
“Any language like that we know is unacceptable and I’m hoping he will apologise for that.”