Police forces are also being told to improve their intelligence systems so that officers questioning suspects can radio their control rooms and apply the “three strikes” rule if the user has already been warned twice.

The Association of Chief Police Officers (Acpo) denied yesterday that the change had been prompted by users escaping with repeated warnings rather than prosecution, but there was a fear that it could become a loophole.

In the guidance, officers are told: “These guidelines do not encourage the same offender being repeatedly warned for possession of cannabis. Where it can be verified that an offender has received two previous cannabis warnings then a further warning should not be considered.”

A similar “three strikes” policy was originally proposed by Acpo in September 2002. The proposal was scrapped before the system of warning for suspects caught in possession of personal amounts of cannabis was introduced.

Under the guidance sent out to all forces in the past few days, children under the age of 18 caught smoking cannabis will no longer be automatically arrested.

The guidelines say that officers should find “less intrusive” ways of dealing with under-18s, such as escorting them home.

Martin Barnes, of DrugScope, the advice group, said: “We welcome the new guidance, which strikes the right balance between personal possession of small amounts of the drug and the offence of supply.

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