Licensees could face a £5,000 fine if they are caught allowing under-18s to use gaming machines in their pubs.
The warning has been issued as an increasing number of local authorities are cracking down with test purchases on gaming machines, according to licensed trade solicitor John Gaunt. However, under law a licensee could also be liable to 51 weeks imprisonment in extreme circumstances.
The latest, Bath & North East Somerset Council, has issued warnings to three pubs out of four it targeted in its latest sting operation.
They were caught allowing under-18s to use their fruit machines.
Solicitor John Gaunt & Partners has advised licensees to review their policy of supervising gaming machines and refresh staff training.
Employees under the age of 18 should be excluded from supervising machines, which should be part of the pub's written policy.
The solicitor explained that a properly drafted and implemented policy could form part of a "due diligence defence" in case of a lapse.
Partner at John Gaunt, Tim Shield said: "Test purchasing used to be more targeted on alcohol, but now the Gambling Commission is encouraging local authorities to act on gaming machines too. Machines are age restricted and may be seen as a soft target. Licensees need to make sure they have a clear view of machines from the bar."
The Publican's Morning Advertiser's legal editor Peter Coulson added: "The test operations are used as a warning. The chances of a licensee being sent to prison are entirely remote. They've always been fined."
A Bath & North East Somerset Council spokesman said: "We are not looking to catch out pubs, only to make sure they realise what's involved in meeting their licence requirements. That is why our first response is to offer a final warning and advice. However, we will not hesitate to take stronger action in the future should that be required."
Gaming machine supplier Games Media managing director Andrew Ludlow added: "If test purchasing is being stepped up, it's crucial that the Challenge 21 scheme that has been so successfully adopted by pubs in relation to alcohol purchases also covers AWP machines."
In March, Durham County Council, launched a series of test purchasing operations on gaming machines.