Confusion reigns over how to apply new laws on cannabis possession, chief police officers will be warned later this week.

Some good, clear guidelines are needed to prevent it going haywire

Tiggey May, report author

The drug is to be downgraded to Class C in October, which means people cannot be arrested for possessing small quantities.

But officers will retain the power of arrest when certain aggravating factors apply.

The new offence of "aggravated possession" is designed to prevent people openly flouting the law by smoking cannabis in front of police officers.

But most beat officers are unsure how this will work in practice, according to an academic who has carried out a study of grassroots opinion.

'Unacceptable disparity'

Tiggey May, co-author of a study on cannabis for the Joseph Rowntree Federation, warned chaos will reign without clear guidelines.

"There is confusion among officers over whether they should be bringing people in over cannabis and when they should not.

"They have not being given clear guidance on what they should be doing."

The Association of Chief Police Officers is in the process of drawing up guidelines on the new law, to be published in November.

But Ms May, who is to address an ACPO conference later this week, will call for an end to the wide variations in the way the law was currently being interpreted.

She told BBC News Online: "The disparity that exists across the country is unacceptable.

"Some good, clear guidelines are needed to prevent it going haywire.

"Justice by geography is never a good idea."

Ms May said the offence of "aggravated possession" was unnecessary as most people would not be "stupid enough" to deliberately smoke cannabis in front of officers.