A schoolboy this morning became the 19th teenager to meet a violent death in London this year.

David Idowu, 14, died at the Royal London hospital from stab wounds inflicted three weeks ago, during an alleged altercation with a group of youths in Southwark, southeast London.

Witnesses said that the row spilled over from nearby football pitches to the spot behind a student halls of residence on Great Dover Street where David was attacked. The youngster is believed to have been stabbed once in the stomach and once in the chest.

He was given first aid by staff at the Sidney Webb House, which houses 700 students from King's College London before being taken to hospital.

A 16-year-old boy has appeared at Camberwell Youth Court accused of attempted murder.

Detective Inspector Bernie Galopin, from Southwark CID, said the investigation will be now handed over to officers from the Homicide and Serious Crime Command.

“Nearly three weeks after the attack on David he has lost his life, and this will now be a murder investigation," said Det Insp Galpin, issuing a fresh appeal for witnesses and information. He added: "In light of David’s death our immediate thoughts are with his family and friends and getting justice for them."

Earlier today a man in his 20s was found dying in the street in Bethnal Green, east London, with stab wounds to his chest and arm.

Police were called to an address in Durant Street, said by locals to be a disused furniture factory which had been taken over by a large group of young hippy squatters before Christmas.

Paramedics took the injured man by ambulance to the Royal London Hospital where he was later pronounced dead.

A second man, also in his early 20s, was treated for head injuries and was arrested after being discharged from hospital. He is in custody at an East London police station, the Metropolitan Police said. It is believed the two men knew each other and the incident was not gang-related.

The recent rash of stabbing deaths in the capital has alarmed police and politicians, and led to calls for tougher penalties for knife crime. David Cameron, the Tory leader suggested today that anyone caught carrying a knife in public without a reasonable excuse should be sent to prison.

Gordon Brown announced last month that anyone over 16 caught with an illegal knife should be prosecuted, rather than escaping with a caution. But Mr Cameron said this did not go far enough, and there should now be a presumption that the automatic punishment was a jail sentence.

According to Home Office figures, 17 per cent of people prosecuted for carrying a knife went to prison in 2006, compared with six per cent in 1996, and the average sentence length has increased by almost a third over the same period.

After a string of high-profile knife murders which have seen 18 young people die in London alone this year, Mr Cameron said knife crime was now a problem of "epidemic proportions" in the UK. He said: "We have to send the strongest possible message that carrying a knife is completely unacceptable in Britain, end of subject.

"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. It is not a minor offence. There is no excuse for carrying a knife when you walk out of your door."

Mr Cameron stressed that police must exercise "common sense" and people carrying penknives for angling or bringing kitchen or garden equipment home from shops should not be prosecuted. "This is about kitchen knives stuffed down the front of tracksuits," he said. "We’re talking about mainly young people carrying knives as part of a culture. That culture has to stop."

Times Recommends

• Times Christmas appeal: Letter from the Editor
• Conferences — what a waste of money
• Schools supremo learns a painful lesson