Gerard Coyne - the challenger to Len McCluskey for the leadership of the Unite union - has been suspended from his post as West Midlands regional secretary.
Mr Coyne - who called for Jews to rejoin the union if he was victorious in a JC interview - was suspended pending an investigation just one day after counting of ballots began.
Following his JC interview, Mr Coyne called in police over a possible hate crime after he was subjected to antisemitic abuse.
Sources say the investigation concerns unproven claims over the use of data by Mr Coyne, and issues relating to the West Midlands mayoral campaign.
The counting of votes had only just begun - with sources today confirming the vote was "extremely close" despite a turn-out of just 12 per cent.
Left-winger Mr McCluskey, an ally of Labour leader Jeremy Corbyn, had been expected to be victorious when the result is revealed on April 28.
But insiders claimed Mr Coyne, who is seen as the anti-Corbyn candidate, was ahead in the ballot of Unite's 1.42 million members.
According to the New Statesman, early returns showed Mr Coyne was ahead in the ballot of the Labour Party's biggest donor.
During the campaign Mr Coyne also asked questions about a deal Mr McCluskey struck with Unite to help him buy a £700,000 flat in central London.
Under the arrangement, Unite contributed £417,000 to the cost of the flat near London Bridge. Mr McCluskey provided the rest of the money himself.
Unite officials insist that such equity share schemes are common, particularly for trade union officials moving to London, that they are properly authorised and generate large profits once the properties are sold.
Standing in for his suspended colleague, Des Quinn emailed Unite officials in West Midlands this morning to tell them disciplinary action had been taken against Mr Coyne.
A spokesman for Mr Coyne told the Politics Home website: “The email said Gerard had been suspended from his duties. We don't know why at this stage and we won't be commenting further.”
