Serve Legal, which provides retailers with an independent audit of compliance with age-restrictive sales legislation, reported that supermarkets passed 87% of its age-verification check tests.
The company conducted some 40,000 test visits in 2015.
Petrol stations (84%) and convenience stores (83%) showed similar levels of diligence.
Scotland has the most compliant retailers in the UK with a 90% pass rate.
However, compliance among online retailers lags far behind their bricks-and-mortar counterparts.
Only 44% of age check tests were passed by the over 1,000 internet retailers tested by Serve Legal in the last three years.
Ed Heaver, director of Serve Legal, said: “The culture of continual, well-managed test and inspection programmes adopted by the majority of UK supermarkets and high-street retailers is undoubtedly driving improvements in compliance.
“We believe this positive commitment to responsible retailing is a significant contributing factor in reducing access to alcohol – and other age-restrictive products – by underage people.
“Many of the retailers we work with build our data into their shop floor training and in-store operational systems to ensure they’re not breaking the law.
“However we are concerned about the delivery practices of online retailers – including supermarkets – where there is little evidence that age verification checks are taking place.
“The law requires that delivery companies request ID from the recipient on the doorstep if the delivery contains any age-restricted products, including alcohol.
“That is simply not happening,” he said.
According to Drinkaware, 43% of young people aged 10-17 say they have had an alcoholic drink, 12% of whom experienced harm as a result.
Serve Legal conducts some 100,000 mystery shopper-style tests on behalf of a range of clients each year, from major multiples to single-site independent retailers.
Serve Legal’s team of shoppers purchase an age-restricted item and record key information about the transaction, including whether ID was requested, a description of the server, a till receipt and other key facts.
All shoppers are young-looking and should be asked to provide ID to complete the transaction.
If a visitor is required to provide official ID – a passport, photocard driving licence or PASS accredited ID – then the shop passes the test.
If the shopper is not asked for ID, or if another kind of ID is accepted as proof of age, then the shop fails.