Kellogg's has been criticised for a Corn Flakes commercial which said eating breakfast cereal made children more alert.
The Advertising Standards Authority said Kellogg's research did not back up the claim.
It found the commercial in breach of the advertising code and told Kellogg's not to show it again without changes.
The advert showed two schoolboys, one of whom ate a bowl of Corn Flakes while the other went without.
A voice-over said: "Research shows when they eat a cereal like ours, kids are on average nine per cent more alert. See if you can spot the difference with Kellogg's Corn Flakes."
Text appeared on screen saying: "Alertness measured by parents, comparing 63 children eating Kellogg's Corn Flakes to 34 skipping breakfast."
An independent expert working for the ASA said scientific evidence generally showed the benefits of breakfast.
But he said Kellogg's research only measured post-breakfast alertness levels for half an hour rather than a whole morning.
And it showed little difference in raised alertness between children who ate cereal and those who skipped the meal.
The ASA said in a statement: "Although the study did show an increase in levels of alertness, it did not conclusively show this was due to cereal. We concluded the study did not support the claim and the ad was misleading."
The ASA rejected complaints from viewers who either thought eating Corn Flakes would not improve concentration or who criticised the cereal's salt content.
Responding to the ruling, a Kellogg's spokesman said there was no dispute with the ASA over the benefits of breakfast.
He said: "While the issue remains an academic debate, we will amend the claim so it is acceptable to both parties."