Five shops in Northampton town centre sold knives to children under the age of 18 in a test purchase operation carried out last week.
Seventeen premises were visited by members of the Northamptonshire Emergency Services Cadets who were working alongside the Northampton Neighbourhood Team.
Of the 17 premises visited 12 refused to sell knives to the cadets. But in five cases, knives were sold to the teenagers.
Shop managers and checkout operators were given "strong words of advice" and warned that further action will be taken in they fail future tests. The police have not named the shops in question.
Section 141A of the Criminal Justice Act 1988 prohibits the sale of knives to those under 18 years of age.
Sergeant Steve Bedford, from the Northampton Neighbourhood Teams, said: “It is reassuring that most of the shops visited successfully challenged the young person attempting to buy a knife.
“However, the only truly acceptable outcome would have been if all of the shops refused to sell a knife, it is very disappointing that so many failed the test purchase.
“All shops should have appropriate policies in place to challenge under-age sales of all forms and ensure that their staff are fully trained, knowing when and how to challenge.”
The test purchase operation was carried out as part of Operation Viper, the force’s crackdown on serious and organised crime, particularly gang-related drug, violence and weapon offences.