Cumbrian MP, John Woodcock, has resigned from the Labour Party.
In his resignation letter, the Barrow and Furness MP said Mr Corbyn would pose a "clear risk to UK national security as prime minister".
And he said that, under Mr Corbyn, Labour was no longer "the broad church it has historically been" and there was little chance of returning it to an "inclusive, mainstream electoral force".
He will continue to stand as an independent, having represented the Cumbrian seat since 2010.
He 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 Labour, saying the allegation was "politically motivated".