However, anyone found smoking reefers in public view — possibly even in their own garden — will still be arrested, as will habitual offenders and anyone aged under 17 found using the drug. Officers will also detain users caught near schools or in areas where local people have complained.

The guidelines, drawn up by the Association of Chief Police Officers, will be used when cannabis is reclassified from a category-B to a category-C drug in January. But reformers were disappointed that the guidance was tougher than expected and the Home Secretary emphasised that cannabis was not being legalised. “It is a harmful drug that is illegal and will remain illegal,” David Blunkett said. But he added: “The change will enable the police to target drugs such as heroin and crack cocaine which cause the most harm.”

Andy Hayman, the Chief Constable of Norfolk and architect of the guidelines, promised “consistency of application” across the country, but cannabis campaigners predicted that there could be a “postcode lottery” with some forces acting more sternly than others — to the extent of people in neighouring streets being treated differently by different commanders in the same city.

The guidance tells officers that although arrests can still be made, “the presumption should be against using this power for simple possession”. But it will be up to officers to decide what amount would constitute “for personal use”.

The guidance also states: “The smoking of cannabis in public view is not in the spirit of reclassification. Such flagrant ignorance of the law has the potential of undermining the illegal status of a controlled drug.” That might mean people smoking cannabis in their own front garden could arrested, for while the property might be private, they could be in public view.

Sebastian Saville, head of Release, which has worked with drug users for 30 years, also pointed out that the guidance did not set out how many times a person had to be stopped before being arrested as a repeat offender, or the distance from a school regarded as “close”.

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