Conservative leader David Cameron has said anyone caught carrying a knife in public without a reasonable excuse should be sent to prison.Prime Minister 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 6 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.The latest victim is a nine-year-old girl who was allegedly robbed at knife-point after she refused to hand over her games console.The crime happened when the girl was walking from a friend's house to her grandmother's house when she was said to have been accosted by the primary school boy.The boy now aged 11, has appeared in court charged with attempted robbery and with the possession of an offensive weapon.The boy is allegedly a member of the Grey Gang in Walthamstow on the Priory Court Estate. Members are known as PYC, the Priory Court Youngsters.The gang's motto is "Bang, bang all day on the blocks."