Half of online retailers tested are breaking the law by selling blades without age checks, according to the trading standard watchdog.
National Trading Standards (NTS) said 50 per cent of online retailers tested between October and January failed to check the age of shoppers buying knives.
The programme also found 12 per cent of retail premises did not age check, while more than 70 per cent did not have theft prevention measures in place.
- Read more: MP admits his GPS knife idea 'probably s**t'
- Read more: Knife offences hit highest level in a decade
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 combating this scourge on society by taking immediate action."
A Home office spokeswoman said the government will support Trading Standards with taking retailers who sell knives to under-18s to court.
She added: “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.”