GMTV's competition phone line provider was fined a record £250,000 for picking potential winners early at a cost of some £20 million to callers, the regulator said today.
Opera Telecom Ltd committed the "worst case" ever seen by regulator Icstis in terms of numbers of callers affected and the amount of money at stake.
The firm picked potential winners before a competition closed, leaving subsequent callers "no chance of winning" despite paying to enter, the regulator said.
All-correct entries submitted after a certain time of day had no chance of winning the GMTV competition.
At least 18 million viewers were charged for entering the GMTV phone-in competition with no chance of winning over a period of nearly four years.
They spent on average more than £1 per call, making the revenue generated by callers "with no hope of winning" more than £20 million, Icstis said.
The £250,000 fine is the highest ever imposed by premium rate services regulator Icstis in its 21-year history.
Opera Telecom Ltd has been ordered to pay a full refund to complainants, although this has been suspended for three months pending an Icstis review of the current refunds scheme.
The firm must also undergo an independent evaluation and make any recommended changes to address "systemic failures within the company".
Failure to do so will result in a year-long ban from running competition services, Icstis said.
Icstis chief executive George Kidd said Opera Telecom had shown a "reckless disregard" for the interests of callers.
"The size of the fine we've imposed reflects the very serious nature of the breach of our rules," he said.
"The consumer harm caused was aggravated by the sheer number of callers who paid to enter the competition but had no chance of winning, the huge amount of revenue that was unfairly generated from these callers, the length of time over which the practice had been going on and the extensive damage caused to the public trust in phone-in competitions."
Ofcom is investigating GMTV's role and will report on its findings "in due course".