A 14-year-old boy has died in a London hospital after being stabbed last month, police said on Monday.

Knifed in Great Dover Street after an altercation with a group of youths on June 17, David Idowu of Southwark is the 19th teenager to die violently in the capital this year.

"Nearly three weeks after the attack on David he has lost his life, and this will now be a murder investigation," said Detective Inspector Bernie Galopin in a statement.

"In light of David's death, our immediate thoughts are with his family and friends and getting justice for them.

"I'd like to take this opportunity to appeal to anyone who has information or who witnessed the attack but has yet to contact the police to come forward."

Deputy Metropolitan Police Commissioner Paul Stephenson has said tackling knife crime is his force's number one priority and has announced the creation of a 75-strong anti-knife squad to target "known gang members and their associates."

Earlier on Monday, Conservative leader David Cameron called for anyone caught carrying a knife to be jailed.

Cameron's custodial sentence demand goes one step further than Prime Minister Gordon Brown who said last month there should be a presumption to prosecute those aged 16 and 17 caught with a knife, rather than merely cautioning them.

"The government should say not just there is a presumption you will be prosecuted if you carry a knife, but that there is a presumption you will go to jail," said Cameron.