A "golden ticket" contest to find a Big Brother housemate broke the rules for such promotions but was not fixed, the Advertising Standards Authority says.
An independent observer should have been present when model Susie Verrico, 43, was picked, the watchdog has ruled.
Channel 4, producer Endemol and Nestle, which hid tickets in KitKats, were told to meet the industry code in future.
The channel welcomed the decision and said it proved the draw was "conducted in accordance with the laws of chance".
Channel 4, Endemol and Nestle claimed it was impossible to position an observer inside the Big Brother compound.
Programme rules prevented outsiders from coming into contact with housemates, the companies said in a joint statement.
The draw was carried out in the presence of 10 contestants under continuous monitoring by a production team and was watched live by five million viewers, they added.
'Undoubtedly' correct
The watchdog said the competition breached clause 35.7 of the Committee of Advertising Practice Code.
But an independent observer had confirmed that 34 individually-numbered balls were used in the draw on 9 June, it added.
And slow-motion footage showed the winning ball was "undoubtedly" stamped with "14", corresponding with the number held by Susie.
A further twist later in the programme's run saw about 3,000 people contacting the premium rate service watchdog Icstis.
It is currently investigating whether its own code was breached when the audience was invited to take part in a vote which led to an evicted housemate being reintroduced to the show.
Some viewers argued that it was wrong to ask them to pay again for a phone call or a text message when the contestants had already been supposedly eliminated from the show for good.
A decision on the case is expected in the first week of October, an Icstis spokeswoman told the BBC News website.
The 13-week series - the seventh to be broadcast in the UK - ended last month and was won by musician Pete Bennett, 24.
The Big Brother format has been successful around the world and the show has now been broadcast in about 40 countries.
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