GMTV was fined £2 million today over misconduct in its viewer competitions.
The fine is the largest penalty imposed against a broadcaster by the watchdog.
The competition picked finalists before phone lines had closed, meaning substantial numbers of people who entered had no chance of winning.
Ofcom has also told GMTV to broadcast a summary of its findings.
Today's fine comes after GMTV's competition phone line provider, Opera Telecom Ltd, was told to pay £250,000 by premium rate phone line regulator Icstis.
Picking potential winners early came at a cost of some £20 million to callers.
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, spending on average more than £1 on each call.
GMTV, which has suspended its premium rate competitions, was found to be in breach of the Broadcasting Code on the fair conduct of competitions.Reuse content