Pubs and shops in County Durham have been urged to adopt a Challenge-25 policy by the council.
The council's Trading Standards team noted a "marked increase" in the number of test purchase failures — where underage children are sent in to try to buy alcohol or cigarettes.
Officers have carried out 38 alcohol and 30 tobacco test purchases in recent months, targeting perceived problem premises.
There have been six alcohol sales and five cigarette sales made to underage volunteers, aged between 15-17.
The failures resulted in a string of fixed penalty notices, written warnings and potential prosecutions.
Premises who failed have been urged to adopt a Challenge-25 policy — asking everyone who appears to be under-25 for ID — rather than the industry standard Challenge-21 scheme.
"Our commitment to step up action against under-age sales has made an early impact and raised awareness among businesses in the county that they must improve their procedures and staff training to prevent illegal sales," said Joanne Waller, the county council's head of environment, health and consumer protection.
"It's clear that staff still do not appreciate how difficult it is to judge the age of teenagers and so should be rigorous in their application of the challenge-25 rule and always ask for identification.
"Follow up visits will be made to all those who made illegal sales and if there are more, prosecution or an application to review a licence will be the likely outcome."
Cllr Bob Young, the county council's Cabinet member for strategic environment and leisure, added: "Our work on under age sales with partner agencies such as the police combined with our drive to improve standards of diligence in the retail trade will protect the future health and well being of our communities," he said.