Figures from retail age checking company Serve Legal showed that shop workers failed to check the age of mystery shoppers buying a knife in 26% of 2,357 test sales in 2017.
Among shops classed as homeware or DIY stores, where 672 tests were carried out, 41% sold the blades to mystery shoppers without checking identification. This also happened in a fifth of supermarkets (21%) where 1,685 test purchases were carried out.
Regionally, the poorest rates were in Scotland and Northern Ireland, which both saw retailers failing 41% of tests. London had the lowest proportion with 18% of tests failed.
The figures were released amid concerns about a rise in gun and knife crime. The latest data published by the Office for National Statistics showed 39,598 offences involving a knife or sharp instrument were recorded in 2017, a 22% increase compared with the previous year, and the highest number registered since comparable records started in 2010.
Firearms-related offences were also up, by 11%, to 6,604 recorded crimes.
In England, Wales and Northern Ireland, it is illegal to sell knives to under 18s, and in Scotland young people aged 16 to 18 can buy cutlery and kitchen knives.
Last year, the Home Office revealed plans to make it an offence to deliver a knife sold online to a private residential address. The buyer will have to collect the knife in person at a location where their age can be checked.
A number of major retailers have also entered into a voluntary agreement to make sure under-18s cannot buy knives, including checking for age identification, reminding customers that they are age restricted products and training staff.
Ed Heaver, director of Serve Legal, said: “Against a backdrop of rising knife crime, news headlines about school-age victims and perpetrators are shockingly frequent.
“Our latest retail test data reveals that despite the principles of the government’s voluntary agreement on underage sales, which many retailers have agreed to adhere to, complacency on the high street could well be contributing to a deadly societal problem, with knives being sold to young people in plain sight.”
The company used young looking 18 and 19-year-olds in the test purchases.