We have to have a separate system for hard core offenders

Sir John Stevens

He said research showed 83% of people living on housing estates in London and Manchester were not prepared to give evidence in court.

"We have identified by research that we have a small hard core of violent offenders in each borough," he told BBC Radio 4's Today programme.

"What we have to do in relation to those is stop their activities."

He said many played the system as they were repeatedly released on bail, sometimes around 10 times before the case got to court, and up to nine months after the offence.

Anonymous evidence

"We have to have a separate system for hard core offenders. It has to be a fast-tracking system that deals with these individuals," he said.

"We have to try to bring in a system whereby witnesses and victims who give evidence can do so in a way they feel they are not going to be threatened or interfered with."

Sir John said some initiatives were working, such as the Crimestoppers scheme which allows people to give evidence anonymously.

He said the research showed 76% of people of London estates were prepared to give anonymous evidence, but such information could not be used in court.

Sir John said the Metropolitan Police were working with local authorities and the Youth Justice Board and backed the police reform proposals.

"Everyone has recognised that we need more resources. We need at least another 2,500 officers in London.

"We do need an auxiliary force that allows us to free up people and officers from the centre of London to go back to duties in the boroughs."