Almost 80% of targeted pubs in Milton Keynes allowed children to buy cigarettes from their vending machines.
A 14-year-old boy was able to buy cigarettes from 11 of 14 tested pubs - which could now face fines of up to £2,500.
The campaign was run to test adherence to the new age limit of 18 for buying tobacco, which came into force in October last year.
The British Heart Foundation estimates that 46,000 11-15 year-olds in England and Wales buy cigarettes from vending machines.
"The publicity surrounding the legislation has been very thorough and in addition we provided advice and information to businesses locally," said senior trading standards officer Allan Martin.
"We will be making further investigations and contacting the businesses concerned."
Cllr Douglas McCall added: "It is extremely disappointing to find that this operation showed such a high number of premises, for one reason or another, allowing a child to buy cigarettes from a vending machine.
"Every time this happens that child is one step closer becoming addicted and as research shows this one foolish act can actually reduce a child's life expectancy."