Vigilant shop keepers have all passed a secret police operation to check up on off-licences.
Lancashire police officers worked with Trading Standards officers to organise a test purchasing scheme at 21 off-licences across Preston one day last week.
Three 15-year-olds tried to buy alcohol in each of the stores as part of the operation.
However, in each of the off-licences, the shop workers challenged the teenagers and refused to sell them drink without proof of age.
Licensing Sgt John Lovick, of Preston Police, said: “Selling alcohol to children is not only against the law but can also put these children at risk from harm.
“I was pleased to see that the premises visited were aware of their responsibilities and took action not to serve our test purchasers.
“We will continue to carry on with our enforcement of the legislation over the coming months.
“We will take robust action against those who pay scant regard to their responsibilities when it comes to refusing sales to under-18s.”
The action comes as part of Operation Cleanslate, ran in Preston over last week in an effort to tackle the causes of anti-social behaviour and crime.