Almost a quarter of shops in some areas are breaking the law on under-age knife sales, with some retailers selling blades to children as young as 12, councils have found.
The Local Government Association (LGA) said irresponsible retailers were putting lives at risk, as the latest official figures showed a 20% annual rise in knife crime in England and Wales.
In test purchases carried out in one area for Trading Standards, an underage teenager was sold a machete, another was sold a lock knife and a 14-year-old managed to buy a nine-inch serrated knife.
In other "alarming" results, seven out of 29 shops, including two major supermarket chains, sold a knife to a person under 18 in purchases undertaken by Devon, Somerset and Torbay Trading Standards, together with police. AIn addition, 96 out of 725 test purchases carried out by London Trading Standards and the police across the capital last year resulted in knives and blades being sold to children as young as 13.
Four retailers, including a major supermarket chain, sold either razor blades or craft knives to a 15-year-old boy and a 16-year-old boy. A trader who sold a four-piece craft knife set to two underage teenagers has been ordered to pay more than £2,000 in fines and costs following a prosecution by Croydon Council.
It is illegal to sell knives to anyone aged under 18.
Those who failed test purchases have either been prosecuted or cautioned, face prosecution or have received warnings and compliance advice on underage knife sales.
The LGA is calling on the retail industry to fund underage test buying operations and work more closely with councils to help improve compliance.
It said heftier fines and tougher sentences were also needed for irresponsible retailers who break knife sale laws.
Simon Blackburn, chairman of the LGA's Safer and Stronger Communities Board, said: "Despite most retailers passing test purchases of knives, trading standards teams at councils across the country are uncovering some shocking abuses of the law.
"Knives are lethal weapons in the wrong hands and it's vital that shops do all they can to prevent them falling into the hands of young people because just one illegal knife sale could have tragic consequences.
"Knife crime has risen significantly in the past year. Clearly there are many different ways that people access knives, whether from home, high street stores or online sales, but we need to make sure that the retail supply of knives is managed robustly across all sales points."
Anyone who suspects that knives are being sold illegally should contact their local council's Trading Standards via Citizens Advice on 03454 040506.