Mayor Boris Johnson has attacked "soft judges" and a criminal justice system that is failing the fight against crime.

Speaking at the annual Crimestoppers dinner last night, he bemoaned the light sentences handed down to those in possession of knives. He said:

"I am worried that where someone is convicted of a knife crime on too few occasions do we get the sentence we need."

The Mayor, who is also chairman of the Metropolitan Police Authority went further, adding:

"I am worried that the criminal justice system is not fit for purpose and is too often letting us down."

Operation Blunt 2 - 4,000 knifes -  1,000 arrests

Operation Blunt 2, which was launched last year to combat the number of teenage killings, has recovered more than 4,000 knives from the streets with portable knife arches and stop and searches — 1,000 people were arrested.

But the courts were failing to take tough action against those caught with deadly weapons, the Mayor said.

Rob Knox killer was freed early

Johnson's attack on the criminal justice system comes after it emerged that the knife killer of Harry Potter actor Rob Knox had been sentenced to four years for GBH but had been freed early on parole before the murder.

Karl Bishop had served 18 months before his release and his licence period ended weeks before he killed the actor in Sidcup in May last year.

Mr Knox, 18, was one of 28 teenagers killed in London last year, including 22 who were stabbed to death.

Sentencing guidelines 'fine' knife carriers.

The Sentencing Guidelines Council recently issued new rules to magistrates to issue fines for first time convictions for carrying a blade in public — despite the maximum punishment of four years jail.

The Tory Mayor's comments tap into national politics - leader David Cameron has pledged that his party will jail any offender caught on the streets with a knife.

Knife crime statistics

At the same time statistics reveal that fewer than one in five people convicted for carrying a knife are sent to prison. The most recent figures show that not one knife offender sentenced for carrying a blade in 2007 received the maximum jail term.

Only 17 per cent of people convicted for possession of knives were sent to jail and of those who were, the majority received sentences of three months or less.

The 2007 statistics showed that 6,121 people were convicted for carrying a blade — 160 fewer than the previous year. However, only 1,060 of them were jailed — fewer than one in five.