Two of the UK's biggest supermarket chains have pledged to improve staff training on knife sales after more than a fifth of cashiers failed to ask for ID in a sting operation.

The pass rate for supermarkets (78%) was higher than that for homeware stores (73%). But the two unnamed supermarket chains, which were subjected to the stings by test-purchasing specialist Serve Legal, have committed to test and tighten sales procedures governing the sale of knives to young people.

Using a team of 18 and 19-year-old testers, Serve Legal made 501 visits to selected supermarkets and homeware stores in the past two months. The young people purchased or attempted to purchase a range of blades, from carving to vegetable knives. A pass mark was given if the shopper was asked to provide official ID in order to complete the purchase.

"The results are not bad," said Serve Legal director Charlie Mowat. "But it's important retailers send out a message of zero tolerance."

Knife crime is once again in the headlines. Last week the Home Office admitted that the number of knife deaths had risen in areas targeted by a major anti-knife crime project.

In June, official figures in the British Crime Survey showed police had recorded 38,082 serious violent knife offences in the previous 12 months, amounting to 104 a day.

However, the 78% pass rate for knives is better than for other age-restricted products. Latest figures from Serve Legal show a 72% pass rate for sales of alcohol and 70% for cigarettes. Retailers were already at the forefront of prevention of knife crime, the BRC said.

"BRC members have worked with the Home Office to sign a six-point commitment on preventing underage knife sales," said food policy director Andrew Opie. "This includes till prompts to remind retail staff to ask for ID at the point of sale and clear public signs that knives will not be sold to underage customers."