TV shows found to be cheating viewers could face criminal proceedings, the phoneline watchdog has warned.
Brainteaser, hosted by Alex Lovell, is normally shown on weekdays
Icstis said any criminal activity found by a review of phone-in competitions would be passed on to police.
The warning came after the regulator met TV bosses to set up a licensing scheme in the wake of allegations some phone-ins were ripping off callers.
Five has dropped quizzes with premium rate services after it was found winners on one show had been faked.
An internal review apparently revealed that a time-limited word quiz on the daytime show Brainteaser sometimes had winners invented if no genuine callers got the right answer.
Bosses at Five, who had no knowledge of the problem, have apologised to viewers.
On Thursday Icstis chairman Sir Alistair Graham unveiled a series of measures designed to restore confidence in TV phone ins.
The licensing scheme, which is expected to be set up within three months, is designed to define who has responsibility for each part of a phone-in programme.
Other measures announced were a systematic monitoring of premium phone services and the publication of clear rules on competitions.
But the first measure to be introduced was the review of current and forthcoming shows.
Sir Graham warned the broadcasters this was not to be taken lightly.
Chairman Sir Alistair Graham said: "If we have any evidence that a possible criminal offence has occurred, we have very close links with the City of London police force and I can assure you we would refer any such evidence for them to investigate."
Icstis also has the power to impose heavy fines for wrongdoing.
The regulator is already investigating six other shows: Channel 4's Richard and Judy, the BBC's Saturday Kitchen and ITV programmes, Ant and Dec's Saturday Night Takeaway, The X Factor, Soapstar Superstar and I'm A Celebrity... Get Me Out Of Here!