Charles Kennedy has challenged the prime minister over what he called Labour's "empty promise" to be tough on crime.

The Liberal Democrat leader highlighted statistics that 25 per cent of teenage boys are now classed as "serious or prolific offenders".

But Tony Blair stated that crime overall under the Labour government is down, not up.

Speaking in the Commons, the prime minister also pointed out that the Audit Commission now do regular reports on youth offending.

"In 1996 they said the current system for dealing with youth crime is inefficient and expensive while little is done to deal effectively with juvenile nuisance," Blair said.

"By contrast in 2004 they said the new arrangements are a significant improvement and a good model for delivering public services."

He admitted youth crime was a problem, but one the Labour Party was tackling.

The prime minister also pointed out that the Liberal Democrats have opposed all measures put forward by Labour to tackle crime, including what he called the "popular" anti-social behaviour measures.

ID cards

Continuing the exchanges, Kennedy stated there was one previous statement by Blair that the Lib Dems "thoroughly endorsed".

Kennedy embarrassed the prime minister by reading a quote in which he called for ID cards to be abandoned in favour of spending more money on frontline policing.

Blair was left on the defensive, saying the situation had now changed.

"Because of the new bio-metric technology we will move to biometric passports... it makes sense at the same time to move to Identity cards," he told MPs.

Blair added that Labour had increased the number of police officers to "record" levels.