A quarter of shops sell knives to minors, research suggests.
Figures from retail age checking company Serve Legal showed that under-18s were able to buy a blade in 26 per cent of 2,357 test sales in 2017.
Among shops classed as homeware or DIY stores, where 672 tests were carried out, 41 per cent sold the blades to young mystery shoppers without checking ID. This also happened in a fifth of supermarkets where 1,685 test purchases were carried out.
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Regionally, the poorest rates were in Scotland and Northern Ireland, which both saw retailers failing 41 per cent of tests. London had the lowest proportion with 18 per cent 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 per cent 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 per cent, 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 age identification, reminding customers that they are age restricted products and training staff.