Gordon Brown has called on the police to adopt zero tolerance towards knife crime after concerns were raised officers were letting offenders off with a caution.
Police officers in 12 high-crime areas will be told to prosecute anyone caught carrying a blade as part of a fresh clampdown on knife crime.
Sir Ian Blair, chief commissioner of the Metropolitan police, has already told his force to prosecute anyone carrying a knife and the prime minister wants this approach to be rolled out to cities including Liverpool, Birmingham and Nottingham.
In an interview with the Sun newspaper, Mr Brown said: "We have to send out a message and reinforce it with action.
"It is completely unacceptable to carry a knife or a gun. Where police have previously been cautioning people, there now has to be a presumption of prosecution."
Last year, 27 young people were killed in London in suspected gang-related deaths and the death toll is on course to exceed that this year. On Saturday a 24-year-old was fatally stabbed in a Brixton greengrocer in an alleged dispute over a stolen orange.
Mr Brown continued: "You cannot be casual or cool about knives. Society cannot cope with people carrying guns and knives and threatening to use them. There are boundaries you cannot cross - and one is this country's zero tolerance on knives."
Under pressure to address the question of supply, Mr Brown said the government would do more to discover how young people obtain knifes, with plans to use teenagers as "test purchasers" to highlight irresponsible retailers.
The prime minister also said he was "very worried" about the effects of violent video and computer games. While stressing he was not advocating censorship, he said the industry must be aware of its social responsibilities.
Home secretary Jacqui Smith will unveil a comprehensive violent crime action plan next month.