An industry chief has urged Tesco to "get its house in order" after four out of 10 stores failed to comply with its policy to combat underage sales.
Tony Payne, chief executive of the Federation of Licensed Victuallers Associations (FLVA) said results from the supermarket giant's in-house audit were "disappointing".
In an internal memo, Tesco revealed just 62 per cent of stores had complied with its Think 21 policy.
Payne said: "A massive company like that, with all its training facilities, has got to make sure its people don't let their guard down."
Tesco staff are meant to ask for proof of age from anyone who looks under 21 before selling them alcohol.
But in the latest round of checks, more than half its superstores failed in the north of the country and 44 per cent in the south. The Tesco Metro range fared better with an 88 per cent pass rate.
The news will be viewed as confirmation that underage problems persist in the off-trade despite initiatives such as ASDA's Challenge 25 scheme.
But Payne said pubs could not be complacent. "The off-trade has the big problems, no doubt about it. But at the same time, the on-trade can't relax. There are lessons for everyone here," he said.
A Tesco spokesman said: "These are not test purchases. They relate to an internal initiative looking at how many customers are asked to provide proof-of-age ID by staff at the tills.
"All the mystery shoppers involved in this process were 18 years old or over and the results give us an insight into the areas where we could further improve our policy and support our staff in tackling underage sales."