High Street retailers have illegally sold knives to children during test purchases conducted by National Trading Standards, the organisation says.
Tesco, Asda, Poundland and Home Bargains sold blades to under-18s at least 15 times each during tests between April 2018 and March 2019.
Tesco and Asda have since updated their checks and further restricted sales.
Poundland said it stopped selling kitchen knives last year, while Home Bargains has not yet commented.
Trading Standards sent in “mystery shoppers” under the age of 18 to carry out the test purchases.
Shop staff failed to prevent the sale of a knife to a child 344 times, equal to 15% of the 2,231 tests carried out by Trading Standards at national chain and independent shops.
It is illegal to sell knives to those under 18, unless it has a folding blade 3 inches long (7.62 cm) or less.
There were 285 killings by a knife or sharp instrument in the 12 months ending March 2018, according to Office for National Statistics (ONS) analysis.
One in four (71) of all victims (285) were men aged 18-24, ONS said.
Lord Toby Harris, chairman of National Trading Standards, 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.
“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.”
Separately, 100 online test purchases were carried out; children were sold a knife on 41 occasions.
Under the new Offensive Weapons Act, retailers will be stopped from delivering knives to residential addresses, in a bid to curb under-18s circumventing age restrictions.
The act, which received Royal Assent in May, is currently under public consultation before the new laws come into force.
Some online retailers already refuse to deliver knives without the courier checking the person’s ID.
An Asda spokesman said: “In April 2019, we became the first retailer to remove all single knives from sale across our stores to help ensure that they do not fall into the wrong hands.
“Whilst we are clearly disappointed with the results from a small number of cases between April 2018 and March 2019, we would like to reassure customers that we have since provided updated training for colleagues.”
The spokesman added the supermarket has “clear Challenge 25 policies”, which require staff to check a customer’s proof of age to ensure they are over 18, when buying a restricted item.
Tesco UK and Ireland chief executive Jason Tarry said: “Tesco takes the safety of our colleagues, customers, and the communities we serve very seriously.
“We have made significant changes to our approach to displaying and selling knives, without taking choice away from customers, including a new two-stage age verification process and removing knives from display on the shop floor.”
A spokesman for Poundland said: “As this body [Trading Standards] is aware, we’re baffled by their numbers.
“They know we stopped selling kitchen knives completely in all our 850 stores last year.”