Jeremy Corbyn's head of policy Andrew Fisher has been suspended from the Labour Party following complaints that he campaigned for a rival party in the general election.
The Labour leader said he continues to have "full confidence" in Mr Fisher, who is understood to remain a member of Mr Corbyn's office.
Labour's ruling National Executive Committee (NEC) has been asked to draw up a report, which could result in disciplinary action against Mr Fisher.
A number of Labour activists and MPs are understood to have made complaints over allegations that Mr Fisher urged supporters to vote for a Class War candidate against Labour's Emily Benn in the Croydon South seat in the May election.
A Labour Party spokesman said: "Andrew Fisher has been suspended from the Labour Party. The NEC will now be asked to authorise a full report to be drawn up with recommendations for disciplinary action if appropriate."
Mr Corbyn said: "I have full confidence in Andrew Fisher and his work. I respect the integrity of the General Secretary's office and trust that this matter will be settled as quickly as possible."