Mr Hellawell led the government's fight against drugs until 2002

Mr Hellawell said he believed Home Secretary David Blunkett would "live to regret" downgrading cannabis from a class B to a class C drug next week.

He told BBC Radio 4's Today programme the move would cause confusion and mean young people were unsure both of the law and of the dangers of cannabis.

But the Home Office said Mr Hellawell had previously accepted the policy.

Mr Blunkett has argued it would focus police efforts on fighting other drugs.

But Mr Hellawell said reclassification had produced "a muddle" and undermined police powers in the field.

The real issue is that the government has given a message that cannabis is less dangerous than it was perceived to be Keith Hellawell

He said: "Cannabis arrests have been steadily coming down within the confines of the existing law.

"Why change it, why cause a problem, why cause confusion, and why - I am sad to say - encourage in some respect greater drug taking, particularly by young people, who don't know where they stand."

But a Home Office spokesman said: "Keith Hellawell supported the home secretary's view that reclassification was necessary when he was briefed on the proposals at a Home Office meeting with David Blunkett in October 2001."

Possession of cannabis will remain illegal but will not be treated as an arrestable offence in most cases, such as home use.

Mr Hellawell accused Mr Blunkett of "driving the agenda" on reclassifying the drug.

'Enormous disservice'

"I do not know why, only he will know, and I suspect he will live to regret it," said Mr Hellawell.

He said there had been no push towards downgrading cannabis when he sat on the principal advisory committee on drugs.

The move would "do this generation and the future generation an enormous disservice", he said.

He added: "The real issue is that the government has given a message that cannabis is less dangerous than it was perceived to be.

"And they have given that message at a time when every medical institution is saying 'we are worried about the dangers, we don't know sufficient about it, and we believe the dangers are even greater than we perceive them to be at the moment'."

Doctors have warned cannabis has been linked to greater risk of heart disease, lung cancer, bronchitis and emphysema, as well as mental health problems.