John Woodcock, the MP for Barrow and Furness, has quit the Labour Party and will sit in the Commons as an independent.
He had been suspended from Labour over a sexual harassment allegation which he denied.
Woodcock, a strong critic of Jeremy Corbyn’s leadership of the party, said the investigation into the allegation was political motivated.
He had been accused of sending inappropriate texts and emails to an ex-staffer between 2014 and 2016.
In a letter to Corbyn on Wednesday morning, Woodcock said he was quitting the party over Labour’s refusal to appoint an independent investigator to oversee the inquiry.
The MP, whose constituency includes a nuclear submarine shipyard, hit out at Corbyn posing “a clear risk to UK national security”.
“The party for which I have campaigned since I was a boy is no longer the broad church it has always historically been,” Woodcock said.
“Anti-Semitism is being tolerated and Labour has been taken over at nearly every level by the hard left.”
“There is little chance of returning the Labour Party to the inclusive, mainstream electoral force my constituents desperately need.
“In these circumstances I can no longer justify engaging in a rigged process to be re-admitted to it.
Woodcock added it was “not credible” that he would receive a fair hearing from the party and would “seek to refer myself to an independent process”.
In response to his resignation letter, cards and a box of roses were left outside Woodock’s office to make fun of his decision.