The Labour MP Jared O’Mara has resigned from the women and equalities committee after it was revealed he had posted a series of homophobic and sexist comments on online forums, about 15 years before he was elected.

O’Mara apologised for a series of derogatory comments about celebrities posted in 2004 earlier on Monday and later resigned from the committee after it emerged further homophobic posts had appeared under his pseudonym on a separate website two years earlier.

The Sheffield Hallam MP said he was deeply ashamed at his online history, unearthed by the Guido Fawkes website. The comments included jokes about having an orgy with members of Girls Aloud, a claim that Michelle McManus had only won Pop Idol “because she was fat”, and a suggestion it would be funny if the jazz star Jamie Cullum were “sodomised with his own piano”.

O’Mara had initially said he wanted to continue to sit on the committee in order to work to confront misogyny. “I understand why they are offensive and sincerely apologise for my use of such unacceptable language,” the MP said in a statement earlier on Monday, issued through his office. “I made the comments as a young man, at a particularly difficult time in my life, but that is no excuse.”

The MP said his views had changed significantly over the past decade and a half. “I have learned about inequalities of power and how violent language perpetuates them,” he said.

“I continue to strive to be a better man and work where I can to confront misogyny, which is why I’m so proud to sit on the women and equalities select committee. I will continue to engage with, and crucially learn from, feminist and other equalities groups so as an MP I can do whatever I can to tackle misogyny.”

But he stepped down from the committee after it emerged that homophobic posts had appeared under his pseudonym on a fan website for the musician Morrissey in 2002, including references to gay men as “fudge-packers” and “poofters”.

One comment read: “I find it funny how some homosexuals think they have the monopoly on being subject to abuse, they should try being ginger … AND disabled!”

The Liberal Democrat peer and former leader of Sheffield council Paul Scriven called the comments “a nasty pattern of sexist language and misogyny” and said he should be removed from the committee. “Having spoken to voters in Sheffield Hallam, they are beginning to question what kind of MP he is,” Scriven said.

The Labour MP Jess Phillips, who sat on the committee with O’Mara, said she was saddened by the comments but said she hoped lessons would be learned. She tweeted:

Tulip Siddiq, another Labour MP who also sits on the committee, said: “Jared’s comments are obviously unacceptable and he’s rightfully apologised for making them.

“Jared has, however, told me personally that he’s been on a very long journey since he posted the comments 15 years ago, made at a troubled time in his life, and I think it takes some self-awareness to acknowledge that and say it publicly. He should get some credit for that.”

At the parliamentary Labour party meeting of backbench Labour MPs on Monday night, O’Mara spoke to colleagues and apologised again, a Labour source said.

Stephen Doughty, the co-chair of Labour’s LGBT PLP group, said he had spoken to the MP about his “totally unacceptable” comments. “I raised my deep concerns directly in person with Jared this afternoon and have received a frank apology,” he said in a statement posted on Twitter.

As more comments from O’Mara were unearthed, pressure mounted from Conservative MPs who said he should be removed from his post on the committee. Mims Davies said O’Mara’s “past language and attitude [was] not at all parliamentary” and her Tory colleague Nadine Dorries claimed it was “safe to say Labour can no longer pretend to be the party of equal opportunities and diversity”.

A spokesperson for the select committee confirmed he had resigned on Monday evening.The Tory MP Sarah Wollaston tweeted after O’Mara’s resignation: “Hard to see how anyone with his views was selected and retains the Labour whip. Sheffield Hallam deserves a by-election.”

The row over O’Mara’s comments comes just days after the Labour MP Clive Lewis apologised for using a misogynist phrase at a Labour party conference fringe event last month after being criticised by several prominent female colleagues.

At a Momentum event, Lewis told the actor Sam Swann to “get on your knees, bitch” which both said was intended in jest.

On Monday, the Commons Speaker, John Bercow, criticised Conservative MPs who had applied for an urgent debate in parliament on Lewis’s comments. Bercow called the request “a wholly absurd and inappropriate application”.