Chief Constable Sir Edward Crew, head of West Midlands Police, criticised the reclassification of the drug as less harmful as sending out the wrong signals to the public.

Sir Edward (l) retires next week

The head of the UK's second largest police force said the new legislation would create extra "bureaucracy" for his officers.

He made his comments at his last official media briefing at police headquarters before his retirement next week.

Under the plans announced by Mr Blunkett last month possession of small amounts of cannabis will no longer be an arrestable offence.

But police would retain the power to arrest marijuana users in certain "aggravated" cases, such as when the drug is smoked near children.

'Wrong decision'

Mr Blunkett will also raise the maximum sentence for dealers of class B and C drugs from five years to 14 years.

Sir Edward said: "I am not very comfortable with the decision if I am frank.

"I think it is the wrong decision in the message it sends out and in terms of implementation it is going to be very difficult".

He argued his officers would face more rather than less bureaucracy to be able to prosecute someone found in possession of the drug.

Hoax calls

But Sir Edward welcomed the fixed penalty pilot scheme for anti-social behaviour introduced in three pilot areas last week as "a sensible way forward" to reduce bureaucracy.

Officers will issue £40 and £80 fixed penalties for offences, such as using threatening behaviour and making hoax 999 calls.

The 56-year-old, who retires next week is being succeeded by Paul Scott-Lee, chief constable of Suffolk Police.

Last year police officers' leaders in the West Midlands passed a vote of no confidence in Sir Edward, accusing him of overseeing a loss of morale among officers.