The Peace Alliance, a London based anti-crime charity, said 29% of the city's youngsters had carried knives.

Now Crimestoppers - the only charity working to solve crime in the UK - is urging youngsters to call them if they know someone who is carrying a blade.

A Crimestoppers spokesman said knife crime was not widespread but was a serious problem in many urban areas.

He added: "In many areas knives have become an alternative to carrying a gun because knives are generally easier to access.

Government bill

"Young people are generally reluctant to tell anyone if they are a victim of knife crime or if they know of anyone who carries a knife.

"The Crimestoppers campaign will encourage young people to ring the 0800 555 111 number if they have any information about people who carry knives and help to get them off the streets."

The government's proposed Violent Crime Bill hope to tackle the problem by raising the legal age of buying a knife from 16 to 18.

Under the new legislation head teachers will also have the right to search students who they suspect are carrying knives or blades into school.

The campaign is being launched at the start of Crimestoppers Week.