MP John Woodcock has quit Labour with an attack on the ‘hard left takeover’ of the party.
The Barrow and Furness MP, an outspoken critic of Jeremy Corbyn, will sit as an independent candidate.
Mr Woodcock was suspended in April amid allegations he sent inappropriate messages to a female member of staff, which he denies.
In his resignation letter the MP tore into Mr Corbyn, saying he would ‘pose a clear risk to UK national security as Prime Minister’.
He accused the party of a dishonest attempt to oust him over the allegations.
His letter brought up the party’s failure to appoint an “independent investigator” to rule on his disciplinary case, and said there was “clear evidence that the process has been manipulated for factional purposes”.
The MP wrote: “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 height of 1980s militancy.
“There is little chance of returning the Labour party to the inclusive, mainstream electoral force my constituents desperately need.”
He personally attacked Mr Corbyn, saying: “I have promised to fight for local jobs, promote a credible alternative government, protect the shipyard and ensure the safety of my constituents through strong defence and national security.
“I now believe more strongly than ever that you have made the Labour Party unfit to deliver those promises and would pose a clear risk to UK national security as Prime Minister.”
A Labour spokesman said: “Jeremy thanks John for his service to the Labour Party.”
His resignation comes as senior MP Dame Margaret Hodge accused Mr Corbyn of being racist.
In a furious tirade, the Dame Hodge branded her party’s leader a ‘f****** anti-Semite’.