Shopkeepers convicted of selling knives to children are more likely to be fined than sent to jail.
In the five years up to 2006, the latest available data, no-one prosecuted for selling knives to under-16s was sent to prison.
Just 71 people have been convicted of selling knives to minors, and the average fine was just £411.16, despite the offence carrying a maximum penalty of £5,000.
The figures, obtained in answers to parliamentary questions by the Liberal Democrats, are for the five years up to the end of 2006.
They do not include convictions since the law changed last year to ban the sale of knives to under-18s.
The research shows that of the 56 people fined in the five years between 2002 and 2006, 75% were fined less than £500.
Liberal Democrat home affairs spokesman Chris Huhne said: "Unscrupulous shopkeepers who sell knives to kids are profiting from the violence on our streets.
"It is unacceptable that so few of them are being punished and those that do are being given such pitiful fines.
"If we are to tackle knife crime, a strong message must be sent to those who ply this deadly trade.
"Fining them a few hundred quid is not going to do that."
