Checkout staff are supposed to ask people who appear to be under 21 for ID before selling them alcohol.
But the memo reveals that only Tesco Metro stores improved their results during the period, with an 88% pass rate.
Tesco Extra stores passed in 64% of cases and Express stores in 63%.
But only 56% of superstores in the South passed the test purchasing initiative and less than half of superstores (44%) in the North.
The memo warns store managers: “If these were external results, stores would be in great danger of losing the ability to sell alcohol…It is vital that all your cashiers and checkout trained staff always ‘Think 21′ when selling alcohol.”
Tesco told Talking Retail: “We dedicate substantial resources to improving our Think 21 policy. This means that anyone who looks under 21 and does not have appropriate ID will not be sold alcohol in our stores.”
And it stressed: “These results 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 under-age sales.”
The warning that stores could lose their alcohol licences is thought to have been a scare tactic, aimed at shocking managers into action.
A Tesco spokesman said the retailer “did very well on underage test purchasing”.
Tesco Express outlets tended to be more problematic than superstores, as staff in the smaller convenience stores had only a handful of colleagues to back them up when refusing underage sales and could feel intimidated, he added.
But one cashier who contacted Talking Retail claimed his store manager was deliberately allowing alcohol to be sold to underaged young people.
“I have not been told to be more careful at the till – no-one has – and I see young people get alcohol all the time,” the cashier said.
