Amusement arcades in County Durham have been criticised for allowing underage gambling on fruit machines.
Durham County Council sent a 16-year-old girl to 13 venues and found she was allowed to gamble at 10 arcades.
The legal age for entering gambling premises is 18 and operators can have their licence revoked or face fines for flouting the law.
Officials will now visit arcades to remind operators of their legal responsibilities.
The arcades failed the test despite receiving a letter from the council warning them of the undercover investigation.
Joanne Waller, Durham County Council's head of environment, health and consumer protection, said: "This is the first time we have used our test purchase powers for this purpose and it is clear that there is a serious issue with non-compliance.
"We are very concerned about how many times our young volunteer was able to gain access to arcades.
"We will now be working with retailers to educate them as to their responsibilities as well as carrying out further test purchases."
Officers from the Gambling Commission were also involved in the investigation.