Chance of arrest depends on police attitude, says report
The study by the Joseph Rowntree Foundation backs proposals by Home Secretary David Blunkett to downgrade cannabis to a Class C drug.
The study - called Times They Are A-Changing - also revealed huge variations across the country in the way offenders are dealt with for possession of cannabis.
It found that some police officers "specialise" in arresting marijuana users while others treat it informally.
It calls for a pilot scheme in Lambeth, London, which treats cannabis as Class C - meaning possession is not an arrestable offence - to be extended nationwide.
Mr Blunkett is expected to reclassify the drug after considering a report from the Advisory Council on the Misuse of Drugs (ACMD).
The report also suggested cannabis possession should no longer be a criminal offence, to save police time.
And it called for setting up a legitimate source of cannabis to prevent users coming into contact with criminal gangs.
As well as the financial benefits for the police, the report also noted social advantages to downgrading cannabis.
Report co-author, Professor Mike Hough, said: "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."
And he dismissed the arguments that arresting someone for cannabis offences could often lead to officers discovering other more serious crimes.
Only 11 custody cases out of 30,000 began with a cannabis offence and led to a serious crime.
Roger Howard, DrugScope Chief Executive, welcomed the report and said downgrading would end the "postcode discrimination" for users.
The research was based on time spent on the police beat and interviews with officers and users.