Police in the UK's worst knife crime hotspots will be told to prosecute anyone caught with a blade, Prime Minister Gordon Brown has said.
Mr Brown told The Sun that carrying knives or guns was "completely unacceptable" and that officers should stop giving offenders cautions.
The Home Office said the crackdown would apply to London, Manchester, Birmingham and Liverpool.
His intervention follows a string of high-profile fatal stabbings.
The issue has been highlighted by the recent deaths of teenagers Faridon Alizada, 18, from Bexley, south-east London, Nassirudeen Osawe in Islington, north London, and Jack Large in Grange Hill, Essex.
Lethal weapons
Mr Brown told the newspaper: "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."
A Violent Crime Action Plan would be unveiled in February by Home Secretary Jacqui Smith, Mr Brown said.
Sir Ian Blair, the Metropolitan Police's chief constable, has already ordered his officers to prosecute all offenders caught with knives, rather than issuing cautions.
However, Camilla Batmanghelidjh, founder of the charity Kids Company, warned that the proposals would not address the underlying causes of knife crime - gang culture and the drug trade.
She added: "I think it's a good idea, but it won't solve the problem.
"The question we need to ask is: why are children carrying knives?"
Mr Brown added that he wanted to use teenagers as "test purchasers" to snare retailers illegally selling blades, and that he was considering outlawing the most lethal weapons.
He also said he was concerned about violent computer games featuring knives.
"No-one wants censorship or an interfering state," he said.
"But the industry has some responsibility to society and needs to exercise that."
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