Off-licences in Lancashire have been caught out selling alcohol to children as young as 15.
Four out of seven Wigan off-licences visited in a trading standards crack down were found guilty of selling alcohol to a fifteen-year-old boy and now face fines of anything up to £5,000 and six months in jail.
Chief trading standards officer at Wigan council, Alan Blundell, said: "Despite our best efforts to hammer the message home about underage sales, it would seem some off-licences are just not listening.
"The operation deliberately targeted these areas because the police and our own complaints records identified them as hotspots for under-age sales."
He said off-licences, like pubs, should be aware of proof-of-age schemes and could put posters up in relevant places.
"There really are no excuses for selling alcohol to people under the legal age. It's inexcusable, unacceptable and just not worth it," he added.
Last month, London off-licences were also found to be flouting the law.
An investigation by Camden council discovered that 80 per cent of shops in the London borough sold alcohol to children as young as 13. Publicans are angry that they often get the blame for underage drinking, when off-licences play a large part.
Trade leaders say this heightens the need for the government to introduce a national ID card, which would make it easier for retailers and licensees to spot underage drinkers.