Black people are not just significantly more likely to be searched by police for drugs than their white peers, but face almost double the chance of being charged if any are found, according to a study of racial disparities in the way drug laws are enforced.

The study showed, for instance, disparities for cocaine possession in London, with 78% of black people charged, compared with 44% of white people. Black people were also almost twice as likely to be charged for possession of cannabis in the capital.

The report, which analysed Home Office data in conjunction with freedom of information responses from police forces in England and Wales, also uncovered what the authors call a "postcode lottery" in the apparent racial basis for drug policing. While black people were just over six times more likely to be searched for drugs nationally, this was significantly higher in some places. In one police area, Dorset, the differential was 17 times.

Previous statistics have shown a likelihood for people from black and other minority ethnic groups to be particularly targeted for police stop and search. However, the new study, by the drug law charity Release and the London School of Economics, argues that drug checks are not only the biggest single reason for search operations but a cause of wider racial inequality throughout the justice system, particularly in that black people appear much more likely to be criminalised for drug possession.

In London, which has the most thorough data for such policing and accounts for half of all the stop-and-search incidents in England and Wales, 12.4% of white people found possessing cannabis in 2009/10 were charged, with almost 75% given a warning.

For black people found with cannabis, 21.5% were charged and 65% warned.

There were also stark differences with the way courts deal with drug possession, with black people being jailed for the offence at six times the rate of white people.

The effect is still more marked in some parts of the capital. Police in the affluent area of Kensington and Chelsea charge black people caught in possession of cocaine seven times as frequently as white people. The differential for charging for cocaine possession was between five and six times for areas including Richmond, Harrow and Wandsworth.

While the government's crime survey shows that drug use appears to be lower in most non-white communities than among white people, the report shows that other minority ethnic groups are disproportionately likely to be searched for drugs.

Asian people were around 2.5 times more likely to experience it in 2009/10, the latest statistic available, with those identifying themselves as mixed race twice as likely. However, these groups did not face a greater likelihood of being charged.

In July the home secretary, Theresa May, announced a consultation over police stop-and-search powers, highlighting the excessive targeting of young black men as a particular problem, as well as a low arrest rate for the 1.2m searches undertaken each year. But the report's authors argue that their findings show there is a need for more fundamental change.

"Despite three decades of calls for police reform, nothing has been achieved," said Niamh Eastwood, executive director of Release. "More people are being stopped and searched than ever before, and the rates of disproportionality are at the very least the same, if not worse, than they were 30 years ago.

"The government needs to change policy, and take drugs out of police hands and treat drug use as a health and education issue."

In London, she noted, about half of all stop and searches are for drugs, against about 10% for knives and less than 1% for guns. "Drugs is what is driving stop-and-search and it's what is driving racial disparity within the criminal justice system," she said.

Her co-author, Michael Shiner, a social policy expert at the LSE, said drug searches were being used by police in some areas as "a form of social control" rather than crime reduction. "There is very little, if any, correlation between crime rates and stop-and-search rates," he said. "To see it as being about crime is to miss the point."

A very low proportion of drug stop-and-searches lead to arrest, noted Eastwood – about 40,000 of the 550,000 carried out in England and Wales in 2009/10, or 7%.

A Home Office spokesman said it would consider the report as part of the review into stop and search. He said: "Stop and search is a crucial tool in the fight against crime but it must be applied fairly and in a way that builds community confidence in the police rather than undermining it.

"We want to see stop and search used only when it's needed, with better community engagement and better search-to- arrest ratios."

Case study: the PhD student



Adam Elliott-Cooper, who is studying for a PhD at Oxford university. Photograph: Sarah Lee

Adam Elliott-Cooper, 26, from north-east London, is studying for a PhD at Oxford University

"I'm mixed race, and I've been stopped quite a bit. After sixth form I took a year off to work and save, and bought a car, and I got used to being pulled over.

"It's happened a lot in my family. My brother goes to Leeds university and he gets stopped and searched a fair amount.

"When I was doing my masters I was also working at a community centre in Plaistow, in east London. It was about 4pm, when loads of school kids come out, and police do lots of stop and searches on the kids as they go home.

"I was coming out of the community centre and I had a stone in my shoe, so I stopped and put my foot up on a wall to take it out. A van load of police officers came round the corner and one of them jumped out. Usually when I'm stopped they say they're stopping you and why, but he said, 'Go on, just give me the drugs. We saw you put it in your sock.'

"I was a bit confused, and panicked a little bit and I took off my shoes to show him they were empty. He was really frustrated that I seemed so bemused.

"They were all so convinced that I'd just put some drugs in my socks. As soon as they realised I didn't have any drugs on me they jumped back in the van and drove straight off. No apology, no receipt, none of the legal requirements."

• This article was amended on 22 August 2013 at the request of the interviewee for personal reasons.