Youngsters employed to detect pubs that serve children will use genuine ID owned by someone over 18 — to catch bar staff who don't make proper checks.
That's under a controversial new approach from Sheffield Police that targets city-centre venues.
A test purchaser would carry valid ID from an adult — probably a relative — that would be shown, if asked, when they order a drink.
If they serve the youngster, it counts as a failure. However, the children would be honest if asked their age. Licensees will be told of failures and successes.
The MA understands that nearby authorities and police are unenthusiastic about the method, believing it amounts to entrapment.
Licensee Nigel Williams, who chairs the 700 venue-strong Sheffield LicenceWatch group, did not object.
"What they are trying to do is to get to the bottom of the problem of false ID, which is a problem in a lot of places," said the Enterprise Inn lessee of the city's Ranmoor Inn.
But Federation of Licensed Victuallers Associations chief executive Tony Payne said: "I would tell [the police] to reconsider. It's a bit low. The police have got to realise they need to work with the trade and not against it."
British Beer & Pub Association northern secretary Lee Le Clercq said Sheffield's new approach goes "a step too far" and is "close to entrapment".
South Yorkshire Police said using an older friend or relative's driving licence or passport is the "most common way" for youngsters to try to enter pubs. "This operation will be run to help and advise licensees and educate doorstaff to tighten up procedures to combat underage drinking," said a spokeswoman.