Alex Cook bought four knives in one hour
Trading standards officers have called for a ban on online knife sales after a machete was sold to a 15-year-old for £1.50 over the internet.
The potential weapon was delivered in the mail in bubble wrap and cardboard to the teenager who was testing under-age sales for trading standards.
The tests found that 214 out of 835 stores in England and Wales sold knives illegally to under-18s.
The problems were more acute online, where 80% sold knives to young people.
"As knife crime remains a problem in many of our towns and cities, it beggars belief that so many traders are still prepared to sell potentially lethal weapons to children," said Ron Gainsford, chief executive of the Trading Standards Institute (TSI).
The law states that it is illegal to sell a knife or bladed article to anyone aged under 18, with businesses and staff facing a fine of up to £5,000 or six months in prison for doing so.
Home Office figures show that knives were involved in 22,151 violent offences in 2007-08.
Knives have become a status symbol among some young people and the increase in searching by police officers means some young people dash into shops to buy knives for "instant arming".
The situation appeared more problematic with sales made online, according to tests by trading standards departments in the London boroughs of Southwark, Lambeth and Greenwich, as well as Staffordshire, Salford and Cardiff.
They found that 58 out of 72 websites selling knives were prepared to sell to children aged under 18, often because they failed to ask the buyer's age.
Sales to teens
Brandon Cook, TSI lead officer for tackling under-age sales, asked his 15-year-old son Alex to test how easy it was to buy knives on the internet using the teenager's debit card.
He bought four, including the machete, in one hour. They started to be delivered within days.
"It was too easy. Anyone can do it, because a lot of people are used to buying things online," said Alex.
His father said that websites offering online gambling were able to put blocks on people under 18 using their sites and the same should be true of websites selling knives.
Issues around underage web-based sales of knives were raised with some major department stores which immediately withdrew them from sale online.
The British Retail Consortium also started a scheme in February that ensures retailers do not sell knives to youngsters. Some 21 major retailers have signed up.
However, the TSI wants the government to ban the sale of knives online.
The Institute is supported by Ann Oakes-Odger, founder of KnifeCrimes.org, whose son Westley, 27, died in 2005 when he was stabbed in the neck in an unprovoked attack at a cash machine.
"As a mother who knows the pain of losing a child, if one life was saved by a ban on the internet sale of knives, it would be worthwhile," she said.
The industry says that work is continuing for a strategy that would allow legitimate consumers to buy age-restricted products.
"We are working towards creating an industry standard so that both online retailers and customers can be assured that we are doing everything possible to ensure that age-restricted products are sold responsibly," said James Roper, chief executive of the Interactive Media in Retailing Group - the online retailing industry body.