Only one shopkeeper has been handed a prison sentence for selling knives to young people in the last 12 years, according to newly-released figures.
Letting a youngster buy a bladed weapon carries a maximum sentence of six months prison and a ?5,000 fine.
Despite this, only one shopkeeper has been jailed for the offence, in 1997, with nobody else jailed or given the maximum fine. The minimum age for buying knives is 18.
In figures shown to the Conservatives in response to written parliamentary questions, it showed that 32 people were sentenced in 2007, with the largest penalty paid ?2,000.
James Brokenshire, the shadow crime reduction minister, said: "It's shocking that so few people are getting punished by the courts for selling blades to kids.
"Even when they do, they get fined a fraction of the ?5,000 that could be given to them.
"Shopkeepers who are caught selling knives [to those who are] under-age should expect to receive a more significant punishment."
Calling for tougher penalties for shops breaking the law, Mr Brokenshire added: "It's all very well Jacqui Smith talking about tough laws on knife sales, but laws are only as good as the enforcement that underpins them.
"On the basis of these figures the Government simply isn't following through on its promises to get knives off our streets."
A spokesman for Jack Straw, the Justice Secretary, said: "Sentences are delivered by judges and magistrates in each case. Our figures show more people are going to jail for carrying knives and that they are getting longer sentences.
"The Government has taken tough action on knife crime ? anyone over the age of 16 caught in possession of a knife can now expect to be prosecuted on the first offence and we have doubled the maximum sentence for possession of an offensive weapon.
"So this Tory publicity hit is actually a direct attack on this country's constitutional arrangements. The independence of the judiciary is a critical foundation of our constitution."
Accusing the Tories of attacking the independence of judges, the spokesman added: "Criticising the judiciary, who do a very difficult job, is counterproductive and unfair, and Labour will not indulge in such grubby tactics.
"Labour believes it is right for Parliament to set the sentencing framework and we have given judges the tools they need to deliver justice. They must be free to make the decisions in individual cases."
The Ministry of Justice figures also showed more people are being sentenced for the offence now than 10 years ago ? between 1997 and 2000 there were only six convictions, whereas in the last four years for which figures are available, there were 118 sentences.
Last month The Daily Telegraph reported that fewer than one in five convictions for crimes involving knives and other weapons led to a jail sentence.
According to police figures, more than a third of knife offences are punished with nothing more than a caution or a fine of as little as ?5.
Earlier this year it was reported that the number of children taken to hospital with stab wounds had risen almost three quarters in the past decade. Last year 22 teenagers were killed in knife attacks in London alone.