Jared O'Mara should be suspended from the party while claims he called a constituent an "ugly bitch" just months before his election are investigated, a Labour MP has said.
Lucy Powell, MP for Manchester Central, spoke on ITV after Labour revealed it had launched an investigation into Mr O'Mara's comments and behaviour.
She told the After The News programme: "One of the key questions you are asked when you become a candidate for the Labour Party - and you have to sign a contract to say this - is there anything in your past that would bring the party into disrepute? And I don't understand how in all honesty Jared could have signed that paper."
She added: "That's why I think he should be suspended while that investigation is taking place."
Mr O'Mara, who won the Sheffield Hallam seat in June, ousting former deputy prime minister Nick Clegg, "categorically denies" the claim made by his constituent Sophie Evans about an incident in March this year.
On Monday, Mr O'Mara resigned from his position on the Commons Women and Equalities Committee in a row over homophobic and sexist online comments posted more than a decade ago.
Newly revealed online comments from the same period show he also made derogatory remarks about the Spanish and Danes.
Labour said the investigation into Mr O'Mara related to the allegations from this year.
"The party is investigating Jared O'Mara MP in relation to comments and behaviour reported from earlier this year," a spokeswoman said.
Ms Evans told BBC Two's Daily Politics programme that some of the comments made to her by Mr O'Mara "aren't broadcastable".
"There were some transphobic slurs in there, but he called me an ugly bitch," she said.
"I just thought 'wow, he's not a very nice man' and just forgot about it.
"I knew that he was involved in politics but I had no idea at the time he was running for MP for Sheff Hallam, which is my constituency."
Ms Evans dismissed Mr O'Mara's claims that he had changed his ways in the years since his online posts, which dated to the early 2000s.
A spokesman for Mr O'Mara said: "He categorically denies the latest allegations."
Party leader Jeremy Corbyn faced calls to remove the whip from the MP, while shadow chancellor John McDonnell said the language allegedly used by Mr O'Mara was "unacceptable".
In the online posts, Mr O'Mara used gay slurs, joked about having an orgy with members of Girls Aloud, claimed Michelle McManus only won Pop Idol "because she was fat" and suggested it would be funny if jazz star Jamie Cullum was "sodomised with his own piano".
Further online comments published by the Guido Fawkes website show Mr O'Mara in a discussion about football referring to Spaniards as "dagos" in 2002 and accusing a Dane of being a "pig shagger".
In an interview with Huck magazine, Mr O'Mara said he had made a "full and unreserved" apology to Labour MPs about the online remarks and felt "deeply ashamed" of his past comments.