WHO Wants To Be A Millionaire? was tricked out of £5million by a cheating gang.
They found out the questions used to pick contestants and gave their own applicants the answers.
And once on the ITV show, the gang gave their contestants a quiz expert for the Phone A Friend option.
Producers believe the scam netted ten per cent of the £50million prize money paid out from 2002 to 2007.
The con was revealed during research into new ITV drama Quiz about Major Charles Ingram, who cheated his way to £1million in 2001 by having a pal cough when the correct answer was read out.
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Quiz writer James Graham said: “One pound in every £10 given away went to this cheating operation.”
Quiz will be screened on Easter Monday.
Ingram is not thought to have been part of the gang.