A teenager was able to use a fake identity card featuring a photograph of a cartoon character from a TV show to purchase alcohol during an undercover operation by trading standards.
The fake ID card featured a photo of Bobby King from the Channel 4 TV show King Of The Hill and showed him to be 17.
But six out of 22 shops in Nottinghamshire served the 18-year-old volunteer despite him showing them the card when he was asked for ID.
And a further seven served him without asking his age, said Nottinghamshire County Council.
The council said it would be issuing advice to all the shops who wrongly served the volunteer during the undercover operation and encouraged all retailers to adopt the Challenge 25 policy.
The policy encourages retailers to ask for ID from anyone who they believe may be 25 or under.
Mick Murphy, the chairman of the council's community safety committee, said: "We take the sale of age-restricted products to children very seriously.
"Although the volunteer was 18, we would strongly recommend that retailers always ask purchasers of goods such as alcohol and cigarettes for identification if they look under 25.
"It is disappointing that around a quarter of the shops tested did not properly check the identification that they asked for before accepting it.
"We are warning shops that we are considering using a child with fake ID following the results of this exercise."