NI21’s deputy leader John McCallister has quit the party.
It follows his decision to launch a probe into the party’s leader Basil McCrea for inappropriate behaviour.
Mr McCrea has denied the claims that emerged in May, on the day of local government and European elections.
Mr McCrea and Mr McCallister set up the party in 2013 after quitting the Ulster Unionist Party.
Mr McCallister said he was staying until he had honoured his duty of care to party staff, which he felt he had now done.
“The NI21 experience has been the most poisonous and toxic experience of my life and at summer recess it is now time to say I’ve met those commitments, honoured those commitments that I said I would,” Mr McCallister said.
“It’s now time to move on and leave the NI21 experiment behind. Certainly it’s with a sense of sadness.”