The Big Brother draw to select a new housemate breached prize promotion rules, the industry watchdog said.
Nestle, Channel 4 and Endemol failed to have an independent observer present when a winning number was picked, the Advertising Standards Authority (ASA) found.
Its investigation followed complaints from 11 viewers about the way model Susie Verrico won her place in the house.
The "golden ticket" promotion involved 100 tickets being hidden inside KitKat bars.
A live lottery-style draw was then broadcast in which one ticketholder's number was picked.
Some viewers thought the draw was rigged and 43-year-old Susie had been pre-selected as the winner.
The ASA rapped Nestle, Channel 4 and Endemol for failing to ensure an independent observer was present immediately before and during the live draw.
"We investigated whether the promotion was conducted under the supervision of an independent observer. On this point, the promotion breached the Committee of Advertising Practice Code clause 35.7 (prize promotions)," the watchdog ruled.
It told the competition's promoters to ensure competitions complied with the code in future. The ASA rejected complaints from viewers who thought the ball numbers were rigged.
In a joint response Nestle, Channel 4 and Endemol said an observer was not present during the draw because Big Brother rules prevented outsiders coming into contact with housemates. They said the draw was carried out in the presence of 10 housemates under continuous monitoring by a production team while watched live by five million viewers.
