Machetes, serrated knives and other blades have been sold to teenagers as young as 13 across London.
In a series of test purchases carried out across London, 96 of 725 stores sold knives to children, according to the Local Government Association (LGA).
This is on top of four Greenwich stores that sold either razor blades or craft knives to 15 and 16-year-old boys.
Similarly, a store that sold a four-piece craft knife set to two teens was ordered to pay more than £2,000 in fines and costs following a prosecution by Croydon Council.
The LGA said the findings are worrying in the wake of latest official figures showing a 20 per cent annual rise in knife crime in England and Wales.
Greater fines and tougher sentences are needed for irresponsible retailers breaking knife sale laws, say the LGA.
Latest figures from the Office for National Statistics show there were 34,703 recorded knife crimes in 2016/17, a 20 per cent increase on the previous year.
Those who failed test purchases have either been prosecuted or cautioned, face prosecution or have received warnings and compliance advice on underage knife sales.
Cllr Simon Blackburn, chair 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.
“With councils experiencing ongoing funding pressures, we are calling on the retail industry to step up and fund underage test purchasing activities and liaise with councils to help improve safety standards and compliance with the law on knife sales.”