Children under the age of 18 were sold knives by four major retailers, National Trading Standards has claimed.
In test purchases carried out between April 2018 and March 2019, Asda, Tesco, Poundland and Home Bargains sold knives to children at least 15 times.
Tesco and Asda have further restricted knife sales since the tests were carried out, with Tesco keeping single kitchen knives in locked cabinets in larger stores, and two members of staff required to check a customer's age.
And earlier this year, Asda announced it would pull single kitchen knives from sale in response to concerns over knife crime.
Poundland, which stopped selling kitchen knives in October 2018, said it was "baffled" by the Trading Standards figures.
Home Bargains has been approached for comment.
Trading Standards teams carried out 2,231 tests at various national chains and independent stores over 12 months, with shop staff failing to prevent the sale of a knife to a child on 344 occasions, equal to 15 per cent.
Chairman of National Trading Standards, Lord Toby Harris, said: "Restricting the sale of knives to children is clearly a difficult issue for retailers, especially those with large numbers of outlets, staff and delivery partners, and I am aware that many retailers are working incredibly hard to train staff and introduce robust procedures to stem the flow of knives to children.
"But let's be clear - it's illegal to sell a knife to a child. Our tests show that it's still too easy for a child to buy a knife."
More than 1,000 children were caught carrying knives in schools in 2018 - with the youngest aged just four.
A total of 1,144 knife possession offences in schools, where the suspect was a child, were recorded in England, Scotland and Wales last year.