Test purchases supervised by Trading Standards officers were made by teenagers in nine shops in the Surrey borough over a four-week period.

The undercover operation was carried out as part of an effort to tackle anti-social behaviour in the area.

"Shop owners really should know better," said Surrey trading standards officer Jennifer Allen.

"They are in a position of responsibility."

'Root cause'

The drive to tackle bad behaviour is part of a three-year Surrey Together project by police and Surrey County Council.

"Our objective is to address anti-social behaviour and an important part of this is to tackle underage drinking, which is often the root cause of youth disorder," said Sgt Terry Hardie.

"This operation was carried out to try and stop the supply of alcohol to youngsters by targeting retailers who sell it to them.

"This is an issue that we treat extremely seriously and we will be carrying out further operations in the area over the coming weeks."