The enormous scale of alleged ballot rigging at the GMB union has shocked senior figures in the labour movement.
In the most recent election of the union's leaders, up to 60,000 ballots were issued to people who had either left the union, retired or died, The Independent understands.
An internal inquiry will hear claims that conspirators gained access to membership records and filled in voting forms on behalf of "ghost" members.
The beneficiary of the alleged scam is said to be Kevin Curran, who has since resigned as general secretary. He has denied any knowledge of or involvement in the alleged fraud.
Mr Curran won the 2003 election for the top job with 60,000 votes to 33,000 for his rival, Paul Kenny, on a turnout of less than 15 per cent.
Up to 6,000 ballots were filled in on behalf of "ghost" members at the AA motoring organisation alone, it is alleged.
Thousands were also thought to have been issued to supposed GMB members at the Millennium Dome, most of whom had moved on two years previously. Others were issued to erstwhile employees at clothing factories which had been closed for years.
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