The Government’s drug policy has been dismissed as a "shambles" and a "mockery" after it emerged cannabis users will continue to escape with a slap on the wrist when the drug is returned to Class B.

The Association of Chief Police Officers confirmed it would bid to retain "cannabis warnings" for people found in possession of dope when it is reclassified. The warnings do not lead to a criminal record.

When dope is upgraded from Class C to B by Prime Minister Gordon Brown – which Whitehall sources say is a certainty – there will be significant differences between the way police treat different drugs in the same category.

Shadow Home Secretary David Davis said: "This will render Gordon Brown’s much-spun U-turn useless and make a mockery of the classification system.

"What kind of message does it send to young people about the danger of this drug that you will still get away with a warning? Clearly the Prime Minister hasn’t got a grip on this policy – that makes him part of the problem, not the solution."

Liberal Democrat home affairs spokesman Chris Huhne said: "The Government’s drugs policy is now in a shambles as senior police officers take the scientific advice more seriously than ministers.

"Gordon Brown should follow the recommendations of the Advisory Council on the Misuse of Drugs (ACMD), as that is the only evidence-based course open to him."

Government sources have said Mr Brown will press ahead with a change in the cannabis laws, despite claims there is no scientific basis for a change.He is set to reject recommendations from a high-powered group of advisers who believe it should stay Class C drug.

A final report from the ACMD was delivered to the Home Office this week.

The experts – including doctors, police, judges and drug counsellors – are understood to have concluded that cannabis should remain in the Class C category.

The Prime Minister is believed to regard the move back to Class B – four years after it was downgraded – as crucial to send a message to young people that cannabis is harmful and linked to serious crime.

An Acpo spokesman said: "We stand by our recommendation made to the ACMD that cannabis should be restored to the category of Class B drug.

"Should the decision be taken to reclassify cannabis to Class B, we would expect to see increased robust enforcement activity particularly in cases involving repeat offenders or where there are aggravating circumstances.

"However, as Simon Byrne, Acpo lead on policing cannabis and Assistant Chief Constable of Merseyside Police, made clear in evidence to the ACMD on February 5, 2008, we would seek to retain the flexibility in dealing with instances of simple possession on the street, including the discretion to issue warnings in appropriate circumstances."

A spokesman for the UK Drug Policy Commission, a charity-funded programme which is examining how illegal drugs are dealt with by the law, said: "It is clear that the role of the classification system has become muddled at the heart of government.

"A review of the wider classification system is clearly needed to examine whether it is fit for purpose.

"Among the options that should be considered we want to see the decision-making on classification taken away from politicians and handed to an expert, independent group."

Danny Kushlick, director of pro-liberalisation group Transform, said: "The classification system was rubbished in its entirety by the Science and Technology Select Committee in 2006, when they said that it was unscientific and not fit for purpose.

"Now Gordon Brown’s need to use a tough-on-drugs stance as political Viagra has brought the Government to a situation whereby different drugs in Class B will have disparate policing policies – some tough, some not.

"There is now no pretence that the cannabis reclassification farrago is anything other than populist grandstanding."

When cannabis is returned to Class B it will be ranked alongside amphetamines, and carry a maximum five year sentence for possession.

However, there is no suggestion that other Class B drugs will be dealt with by "warnings".

A Home Office spokesman said: "Our message has always been that cannabis is an illegal and harmful drug that should not be taken.

"While evidence shows that cannabis use is falling across all age ranges, we are concerned about stronger strains of the drug.

"That is why we asked the ACMD to undertake a review of cannabis classification."

> "Has Gordon Brown been smoking something?" - read criminologist James Treadwell's blog