MANY police officers have effectively decriminalised possession of cannabis by turning a blind eye to the offence, according to a major study published today.

But, in a unique street-level survey, researchers also found that a small minority of officers "specialise" in cannabis offences - and that rookies were encouraged to make arrests as a way of "learning the ropes".

The widespread inconsistencies in the treatment of cannabis possession emerged from the study, which has been unveiled by the York-based Joseph Rowntree Foundation.

Experts from the criminal policy research unit at London's South Bank University carried out fieldwork in contrasting areas of the country - including spending time studying the practices and attitudes of Cleveland Police in relation to cannabis offences.

The force was one of eight to be examined before researchers narrowed their efforts down to four districts, observing what happens "on the beat", as well as interviewing police officers and those they arrest.

The report says the chances of being arrested depends on the force area where an offence is discovered - and on the attitudes of individual officers.

It found that:

* A third of officers in case study areas said they always arrested people found possessing cannabis. There was evidence of "specialisation", with three per cent of officers who made arrests for possession accounting for 20 per cent of the total.

* Two out of three officers had dealt with cannabis informally at some stage in their career. Many had effectively decriminalised cannabis in their everyday working practices.

* The cost of policing cannabis in England and Wales is roughly £38m a year, absorbing the equivalent of 500 officers working full-time.

The report claims that Home Secretary David Blunkett's plans to reclassify cannabis as a class C drug would produce cost and efficiency savings.

The loss of a very small number of cannabis arrests that lead to serious offences being detected would be offset by more effective use of police time, it said.

Co-author of the report, Professor Mike Hough, said: "A lighter enforcement regime is, in our view, unlikely to lead to significant growth in the number of young people who use cannabis.

"The non-financial benefits could be large, since reclassification is likely to remove some of the friction between police and communities that currently hinders co-operation in tackling more serious crime."