Trading Standards discovered that some branches of well-known national retailers were breaking the law.

In random tests, five youngsters aged 13 and 14 were asked to try to buy a knife - in 15 out of 20 shops they succeeded.

It is illegal to sell a knife to a minor.

Legal advice

Trading Standards officials are warning shopkeepers that they can be prosecuted if they are found selling knives to anyone under 16.

Folded pocket knives with blades of less than three inches in length and cartridge-style razor blades are not covered by the legislation.

Tesco, Sainsburys, Morrisons and Woolworths, as well as smaller hardware stores, were among the outlets selling the knives to youngsters.

Trading standards officers seized a haul of knives, including kitchen knives, craft knives and carving sets with blades ranging from three to eight inches long.

All of the knives could have been used as weapons.

Councillor Walter Burrows, Derbyshire County Council's cabinet member for Public Protection and Transport said: "Nationally there have been reports of knives being carried by young people and we want to make sure we are doing all we can to prevent incidents occurring locally.