Union officials have accused Trading Standards of being too indiscriminate with underage sales stings and not doing enough to catch repeat offenders.

Speaking at the annual delegate meeting last week, Usdaw general secretary John Hannett called for test purchasing to be more targeted and intelligence-led and less geared towards notching up prosecutions of shop staff who may only have made one mistake.

"Current legislation is too harsh in respect of the individual employee and in effect criminalises people for making an error in judgement," he said.

Most staff weren't deliberate offenders and many felt under pressure to make a sale, he added, citing an Usdaw survey revealing that two thirds of shop workers had been subjected to verbal abuse when asking for ID and one in five had not been satisfied with the support they received from their employer.

Usdaw also criticised inconsistencies in rule enforcement. Tests are conducted by local Trading Standards teams using a non-binding code of practice. Some teams have quotas for the sting operations they conduct while others tell the underage volunteers involved to lie about their age or use fake ID.

However, Serve Legal, which carries out 60,000 test purchases for retailers each year, said it was important to be able to target a variety of retailers.

"Shopworkers are the interface between the underage purchaser and the retailer," said director Charlie Mowat. "They have to take responsibility for not serving underage people. Saying 'we just want serial offenders to be targeted' is missing the point."

The government announced in March that it wanted to simplify more than 20 pieces of legislation that regulate age-restricted sales. It also plans to abolish or simplify current licence requirements.

The Local Better Regulation Office has been holding industry seminars to develop a new test-purchasing code. It will begin a public consultation in June and publish its findings by the end of the year.