Gerard Coyne - the man challenging Len McCluskey for the leadership of Unite - has been suspended from his job as a regional official with the union.
The move comes as counting is about to get under way in the union's leadership ballot.
Mr Coyne, who is seen as the anti-Jeremy Corbyn candidate, has been suspended as West Midlands regional secretary pending an investigation.
He is not thought to have been suspended as a union member.
Mr Coyne's campaign was recently warned by the union's acting general secretary that some of his campaign literature contained potentially defamatory statements about Mr McCluskey.