More than one third of County Durham retailers visited in a sting operation sold knives to an under-age girl.
The 16-year-old volunteer visited 33 outlets and was able to purchase items including Stanley knives, kitchen knives and an axe.
The purchase was allowed, even though letters had been sent out warning that test purchases would be taking place.
The businesses which made the illegal sales are being investigated, and could be cautioned or fined.
The operation, by police and Durham County Council trading standards officers, was carried out in Peterlee, Durham, Consett, Stanley and Chester-le-Street.
Joanne Waller, the council's head of environment, health and consumer protection, said: "We are very concerned by how many times our young volunteer was able to buy potential weapons, and how reluctant some sellers were to operate their checks and procedures.
"We shall be extending our education and testing of retailers to ensure that the situation improves."