Media playback is unsupported on your device Media caption Ken Clarke: "Serious knife crimes will get serious prison sentences, but we're not setting absolute tariffs"

A Conservative election pledge that anyone caught carrying a knife could expect a jail term will not be implemented, the BBC has been told.

Justice Secretary Ken Clarke said he would put sentencing policy in the hands of judges, not newspaper pundits.

But he said those guilty of using a knife would face a "serious" jail term.

His comments came ahead of a Green Paper aimed at reducing jail numbers in England and Wales through changes to sentencing policy and rehabilitation.

Asked by BBC political editor Nick Robinson whether people caught carrying knives could expect a lesser punishment, Mr Clarke said ministers would not insist on "absolute tariffs".

It means that, as at present, someone caught carrying a knife may not face a custodial sentence, and may be cautioned instead.

In the Conservative election manifesto, the party said four out of five people convicted of a knife crime did not go to prison and they would send a "serious, unambiguous message that carrying a knife is totally unacceptable".

The document said: "We will make it clear that anyone convicted of a knife crime can expect to face a prison sentence."

But Mr Clarke was asked whether people convicted of carrying a knife could get a caution or community service.

He told the BBC: "We're not setting out absolute tariffs for particular things. What happens is pundits or newspapers suggest levels for particular forms of crimes ... Parliament in its wisdom enacts them - it doesn't work.

I'm not in favour of absolute rules. I'm in favour of actually allowing judges to see how nasty the offender is Ken Clarke, Justice Secretary

"Anybody who is guilty of serious knife crime will go to prison but I'm not in favour of absolute rules. I'm in favour of actually allowing judges to see how nasty the offender is, see what the offence was, see what the best way of protecting the public from him is."

"I'm more interested in actually, will we stop this man doing this again in future?"

Mr Clarke said he wanted to drive down inmate numbers through a "rehabilitation revolution" to reduce the numbers of people who re-offend after leaving prison.

He said that meant tackling problems like drug and alcohol abuse, mental health and lack of jobs for ex-offenders. The justice secretary says he wants private firms to be paid by results to run rehabilitation programmes.

The BBC understands that the Green Paper on Tuesday will set out plans that those who plead guilty immediately will receive discounted sentences and that there will be fewer indeterminate sentences - where there is no fixed release date.

All the main parties now agree on the need for what Mr Clarke will hail tomorrow as a rehabilitation revolution to stop prisoners re-offending Nick Robinson, BBC political editor Read Nick's thoughts in full

Last year about 4,600 people were admitted to hospital in England with knife wounds. In the 12 months to June 2010, there were 29,981 knife crimes in England and Wales recorded by the police - a drop of 7% from the previous year.

In July 2008 David Cameron said a "presumption to prosecute" did not send a strong enough signal and there should be a "presumption to prison" for knife crime.

He told the Sun: "This is about kitchen knives stuffed down the front of tracksuits... We're talking about mainly young people carrying knives as part of a culture. That culture has to stop."

During the election campaign he was joined by Brooke Kinsella - the former EastEnders actress whose brother Ben was stabbed to death in 2008 - who has since become a government adviser and is reviewing schemes aimed at tackling knife crime in England and Wales.