Tests last year found three in four shops sold blades to children, breaking the Criminal Justice Act 1988.

Officers were so shocked they stopped checks to educate traders of the risks of having knives on the streets.

Councillor Walter Burrows, Derbyshire's cabinet member for public protection and transport, said: "The law is there to prevent injury and harm."

Undercover tests

Retailers are being offered guidance by trading standards officers to prevent the underage sale of solvents, knives and other bladed objects.

They are outlining the law ahead of more undercover tests to see if retailers will sell products to youngsters who are too young to buy them.

There will be checks on knives and solvents, plus, for the first time, tests on aerosol sales.

Regular checks for underage sales of other age-restricted goods such as cigarettes, alcohol and fireworks are also undertaken by the council.

Sales staff are being advised to ask for proof of age if they are not certain the purchaser is old enough to legally buy an age restricted product.