Tesco has been fined £10,000 after it was caught selling a bottle of wine to a 16-year-old boy.
The store was caught in an undercover sting by police.
It also emerged that the Tesco Express store in Dobbins Place, Crawley, had been illegally selling alcohol for two and a half months without a Designated Premises Supervisor - effectively a licensee.
The store had sold alcohol without a DPS since former manager Daniel Le Roux left the company on July 29 and until the sting on October 13 last year.
In a landmark case at Crawley Magistrates Court yesterdayMON, Tesco admitted a charge of exposing alcohol for unauthorised sale by retail.
District Judge Roger Eade fined the supermarket £10,000 - half the maximum £20,000 - and ordered Tesco to pay £1,200 costs.
He said: "For almost two and a half months alcohol was being sold without proper supervision. It is not good enough, particularly for a store of the sort of standing that Tesco has.
"The public are very concerned about sales of alcohol to people under age, and very concerned that alcohol should be sold in a responsible way."
The court heard the Tesco Express store sold an average of £4,000 worth of alcohol a week.
Craig Baylis, defending, said the responsibility lay with Tesco's area manager, who had faced disciplinary action. He also said the company's procedures had since been changed to prevent a recurrence of the offence.
The case marks the first time a supermarket has been successfully prosecuted for flouting the DPS law under the Licensing Act.
Speaking outside the court, Jean Irving, licensing and public safety manager for Sussex Police, said: "This sends a clear message out that the courts are taking this sort of thing very seriously.
"Selling without a designated premises supervisor is the equivalent of someone walking down the street selling booze - it is illegal.
"This isn't the first case we have come across. For some reason supermarkets don't deem it that important. The police and Crown Prosection Service worked very closely to achieve what we believe is an excellent result.
"Alcohol is a drug and it must be sold responsibly. Hopefully others will learn from this."
The Dobbins Place branch and the Tesco Express store in Downland Drive, Crawley, are currently appealing against a 28-day ban on selling booze.
The supermarket's appeals against the license suspensions, brought by Crawley Borough Council, are due to be heard at Horsham Magistrates Court., on Monday.
The Argus recently revealed the Tesco Express in Broadwater Street, Broadwater, Worthing, was banned from selling alcohol for four weeks from July 2 after staff were caught supplying booze to children on three separate occasions.
Recently published figures show that around 25 per cent of 15 to 16-year-olds drink regularly, with 23,000 crimes nationwide attributed to drunk children.
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