Labour has suspended Jeremy Corbyn's head of policy Andrew Fisher following complaints he campaigned for a rival party in the general election.
The Labour leader said he has "full confidence" in Mr Fisher as an investigation is launched into the allegations, which could result in disciplinary action.
MPs and activists are believed to have complained to the the ruling National Executive Committee over claims Mr Fisher told supporters to vote for a Class War candidate instead of Labour's Emily Benn in the Croydon South seat.
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."
Ms Benn, granddaughter of Labour stalwart Tony Benn, made a formal complaint about Mr Fisher after he sent a tweet which read "FFS if you live in Croydon South, vote with dignity, vote @campaignbeard" - a reference to the Class War parliamentary candidate Jon Bigger.
Under Labour's rules, any member who "supports any candidate who stands against an official Labour candidate... shall automatically be ineligible to be or remain a party member".
Mr Fisher wrote to Labour's general secretary Iain McNicol to "completely and unreservedly" apologise for the tweet, which he said had been "misinterpreted".
He wrote: "I obviously do not and did not support Class War in any way, let alone in an election. I was and am committed to the election of a Labour government and a successful and united Labour Party."
Mr Fisher continued: "I accept that the tweet has been misinterpreted and has caused embarrassment and understandable upset among party members, which I regret.
"I wish to completely and unreservedly apologise for this tweet. I have now closed my social media accounts and assure you there will be no repetition of such activity in the future."
Additional reporting by the Press Association
