Retailers have been warned they must take immediate action to help prevent knife crime after testing found many were selling blades without making the required age checks.
National Trading Standards (NTS) said 12% of the retail premises it visited between October and January failed to check the age of shoppers buying knives, increasing to 50% of online retailers.
It is illegal to sell a knife to anyone under 18 years old.
The programme also found that more than 70% of the retailers visited did not have theft prevention measures in place for knives.
NTS is not naming the retailers which failed the checks due to potential enforcement action, but said they included a number of well-known high street and online brands.
The NTS warning comes amid a knife crime “epidemic” following a spate of fatal stabbings.
Official figures just released show the number of criminals caught with knives or dangerous weapon has hit its highest level in nearly a decade.
Last year, more than 21,000 offences of possessing or making threats with blades or offensive weapons resulted in a conviction or caution in England and Wales.
One in five of the culprits was under 18.
NTS chairman Toby Harris said: “As we have tragically seen in recent weeks and months, knife crime is one of the most serious issues facing our country, and we have sadly witnessed its fatal consequences.
“Our programme with the Home Office has only been running a few months but already evidence has shown that knives appear to be being sold illegally to young people. We call on retailers to play their part in combatting this scourge on society by taking immediate action.”
A Home Office spokeswoman said: “This Government is determined to prevent access to offensive weapons.
“It is already illegal to sell knives to under-18s and it is vital that retailers continue to play their part. We have and continue to work with high street retailers to put in place a set of voluntary commitments to prevent the under-age sales of knives in their stores and online.
“As part of the Serious Violence Strategy, we are supporting Trading Standards through a new Prosecution Fund to help them to take rogue retailers who sell knives to under-18s through the courts.
“In addition, our proposed Offensive Weapons Bill will also introduce legislation that will make it harder than ever for young people to purchase knives.”