John Woodcock has quit as a Labour MP and will now sit as an independent in the House of Commons.
The Barrow and Furness MP, a long-time critic of Jeremy Corbyn, was suspended in April over claims he sent inappropriate messages to a former female member of staff.
He has denied the claim, which is being investigated by the party.
In his resignation letter, he said the party had been "taken over by the hard left" and "tolerated" anti-Semitism.
He has represented the Cumbrian seat since 2010, winning it with a 209 majority last year.
My letter to Jeremy Corbyn resigning from the Labour Party. I will continue to serve the people of Barrow and Furness as an independent MP pic.twitter.com/8PJtgurK3X— John Woodcock (@JWoodcockMP) July 18, 2018
A Labour spokesman said: "Jeremy thanks John for his service to the Labour Party."
Mr Woodcock, who is on the right of the party, had the Labour whip withdrawn earlier this year amid claims he sent inappropriate messages to a former female member of staff.
He has said he does not accept the charges and was fully co-operating with an internal inquiry.
Announcing his resignation from the party, he accused Labour of failing to appoint an independent investigator to rule on the disciplinary case.
He suggested the process was being "manipulated for factional purposes" and he had seen e-mails showing senior party figures wanted to stop him standing as a candidate again because of his views on the leadership.
In a wide-ranging attack on Mr Corbyn, he said Labour was not a "credible" alternative government and its leader would pose a "clear risk" to the UK's security if he became prime minister.
"I have promised to fight for local jobs, protect the shipyard and ensure the safety of the my constituents through strong defence and national security.
"I now believe more strongly than ever you have made Labour unfit to deliver those objectives."
Mr Woodcock, who asked a question about Northern Rail during Wednesday's session of Prime Minister's Questions, said he was committed to continuing to represent his constituents in Parliament and was prepared to "work with the government when it is trying to do the right thing".
He added: "The party for which I have campaigned since I was a boy is no longer the broad church it has always historically been.
"Anti-Semitism is being tolerated and Labour has been taken over at nearly every level by the hard left, far beyond the dominance they achieved at the 1980s militancy."
'No big deal'
Labour has rejected claims its disputes resolution process has become politicised, pointing out that the MP's resignation means the investigation into alleged harassment cannot be concluded and the complainant will be denied their right to have their claim heard.
Asked whether there should now be a by-election, a party spokesman said "basic rules of democratic accountability" dictated that when MPs "abandoned" the platform they were elected on, the public should have their say.
And Unite general secretary Len McCluskey, who has called on critics of Mr Corbyn to shut up or get out, said the MP's resignation from the party was "no big deal".
"He resigned from Labour's values a long time ago."
His resignation comes a day after Labour's ruling National Executive approved a new code of conduct on anti-Semitism, which has been criticised by Jewish leaders and some of its own MPs.
The document, drawn up after protests by Jewish groups against Labour's handling of the issue, states anti-Semitism is racism but does not fully repeat the International Holocaust Remembrance Alliance's definition.
Labour said the code, drawn up following the 2016 Chakrabarti inquiry into anti-Semitism, addressed concerns about alleged anti-Semitic remarks by activists and better reflected the views of Jewish community groups.