Police should not count minor drugs possession when measuring their performance so they are not forced to unduly target drug users, a study has found.
The number of cannabis possession offences recorded by police almost doubled between 2004 and 2011, while reported use of the drug fell by a quarter, the London School of Economics study also found, while the number of police stop and searches for drugs more than doubled.
The study, by Dr Michael Shiner, also found stop and searches for stolen goods fell from two-fifths to one fifth of all stop and searches in the same period.
He also identified a surge in police targeting people after cannabis was reclassified a Class C drug in 2004 - something that was reversed in 2009 - because officers also received new powers to issue on-the-spot street warnings and fines, adding this "created a fast-track to achieving targets at a time when police performance was under considerable government scrutiny”.
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Dr Shiner's paper, published in The International Journal On Drug Policy, said removing cannabis possession would also help officers focus on more serious crime, rather than be driven to target such minor offences.
It follows commitments from several police forces to not target people who possess small amounts of the drug.
“Many otherwise law-abiding, mainly young, people are still being criminalised to the detriment of their future; and drug policing continues to be disproportionately targeted at minority ethnic communities,” Dr Shiner writes.
“On a more optimistic note, the introduction of cannabis street warnings has inadvertently shown that police behaviour is malleable and responds to incentives.
"Perhaps the best administrative decision that could be made in the short-term is to remove drug possession from police performance indicators. Then we might get a police service that concentrates on crimes that cause most harm.”
The potential impact of police performance targets on force priorities was revealed in 2012, when Kent Police was at the centre of a scandal about how its targets were forcing officers to pursue more minor crimes to boost the number of crimes solved.
Ian Pointon, the chairman of Kent Police Federation, said at the time that officers had to alternatively keep crime figures down but the number of solved offences high.
"When we want to reduce crime, (we) deal with simple drug possession in a more pragmatic way - seize the drugs for destruction but use (our) discretion and give a verbal warning," he said.
"It cannot be right that someone in simple possession of cannabis gets a criminal record based on where Kent Police currently sits in its performance cycle."
Police in Durham have said they will not prioritise people who grow a small amount of cannabis
Last month, Durham's police and crime commissioner Ron Hogg said his force would not target people who grow a small amounts of cannabis for their own use.
He said: "In low level cases we say it is better to work with them and put them in a position where they can recover."
Derbyshire, Dorset and Surrey police forces later made commitments of their own to do the same.
In reaction to Mr Hogg's comments, a spokesperson for drugs reform advocates Release told HuffPost UK: "Our research into over 20 jurisdictions around the world has shown that the decriminalisation of personal possession and use does not lead to an increase in prevalence as some fear it will.
"In the UK, we are handing out criminal records to countless numbers of young people for simple possession offences, something which is hugely detrimental to their future employment and education opportunities. This needs to stop."