Asda is piloting a Challenge 25 policy for the sale of alcohol in 10 Scottish stores, with anyone who looks under 25 being asked to show ID. If successful, the scheme will be rolled out across the UK.
"This is about giving colleagues more tools when dealing with the problem of IDing those not entitled to buy alcohol," said retail director Andy Clarke.
Asda became the first retailer to introduce Challenge 25 after it received "alarming" results from a survey, he said.
The supermarket took a group of 16-year-olds on to the streets of London and Glasgow and asked shoppers to guess their age. Nearly three quarters said the children were 18 or over, and a quarter thought they were over 21.
The Scottish Executive has now empowered all local authorities in Scotland to test-purchase for alcohol sales using minors. The roll-out follows a trial in Fife, in which 17% of the 600 premises tested sold booze to underage volunteers.