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.