Tesco, Asda and Poundland have been found selling knives to children at least 15 times over the past year, according to National Trading Standards which conducted test purchases.
Mystery shoppers under the age of 18 conducted 2,231 tests on multiple and independent retailers, with 15% allowing the sale of a prohibited knife.
Of the 344 failed tests, 174 were failed by independents and 170 by multiples.
AgeChecked chief executive Alastair Graham told betterRetailing: “Retailers need to ensure that compliance does not come at the expense of a seamless experience for legitimate customers.”
Independent retailers on tight budgets should take advantage of free training to help prevent selling knives to children and other age-related sales: Free training to tackle underage sales