Britain's most senior police officer today urged parents to confront their children at home in a bid to halt the knife-carrying culture.

Metropolitan Police Commissioner Sir Ian Blair said the fight against youth crime should begin with parents challenging teenagers over whether they were carrying weapons.

Another weekend of violence in London saw the death of Harry Potter actor Robert Knox, 18, the 14th teenager to be killed in the capital this year.

Sir Ian said that the knives seized by police were typically of the type that could be found in any kitchen drawer. He said: "Parents have a duty, they need to be asking their teenagers 'Are you involved in knife carrying?'

"In the same way as you should question your child about drugs and alcohol you should also be clear to them that carrying a knife outside their home is likely to lead to serious trouble and tragedy.

"I think every child has a right to tough love. I think parents do have a duty to find out if their child is involved because if they are they are in significant danger."

Sir Ian gave details of a tough police crackdown on youth violence and gangs. Operation Blunt 2, launched in the past two weeks, has seen 200 arrests and the seizure of more than 130 weapons, mostly knives.

Police are using stop and search tactics and deploying metal detector arches to catch children with weapons.

The operation is being run in three boroughs - Southwark, Lewisham and Croydon. Police plan to extend it to the 10 boroughs with the highest number of knife attacks over the next two weeks.

The operation has the support of detectives from specialist crime units which tackle more serious gangs. Over the weekend they seized a machine gun, three other guns and a silencer in one raid.

Sir Ian welcomed comments by a senior judge who said those caught carrying knives should face jail. He said there had been "conflicting messages" on the penalty for those found with knives.

Sir Ian said: "Young people are not only the problem, they are also part of the solution and the vast majority of young people are law-abiding. Our message is that young people who carry knives are likely to be arrested and if they are arrested they are likely to be prosecuted and then they are likely to face a custodial sentence."

Deputy Assistant Commissioner Rose Fitzpatrick said officers were also stepping up visits to home addresses to warn parents of their children's activities. She said: "Even a small knife - even the sort of thing you would pick out of the kitchen drawer - can kill someone."

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