CANNABIS has become far stronger and much more expensive in the past decade, new research claims.

Researchers say the class B drug – the most widely used illegal substance in the UK – has become twice as potent and may pose a bigger danger to users.

However, experts say its illegal status means changes in its make up have not been studied in-depth, leaving policy-makers unable to take steps to control this.

The claims come from Dr Tom Freeman of the Addiction and Mental Health Group within the Department of Psychology at the University of Bath.

Freeman, the lead author in a new report out today, found cannabis potency has doubled across Europe in the past 11 years.

Concentrations of delta-9-tetrahydrocannabinol or THC – the main psychoactive constituent of cannabis – in the herbal form of the substance increased from 5% in 2006 to 10% in 2016.

For cannabis resin, THC concentration rose from 8% in 2006 to 10% in 2011 before jumping again to 17% by 2017.

Meanwhile, the prices for both forms have risen significantly over the study period.

Cannabis resin typically contains cannabidiol (CBD) in addition to THC.

When present in cannabis, CBD may offset some of the harmful effects of THC such as paranoia and memory impairment.

However, the study – which is the first of its kind and used data from 28 European Union member states as well as Norway and Turkey – found new resin production techniques in Morocco and Europe have increased levels of THC but not levels of CBD.

And Freeman said: “CBD has the potential to make cannabis safer, without limiting the positive effects users seek.

“What we are seeing in Europe is an increase in THC and either stable or decreasing levels of CBD, potentially making cannabis more harmful.

“These changes in the illicit market are largely hidden from scientific investigation and are difficult to target by policy-makers.

“An alternative option could be to attempt to control THC and CBD content through regulation.”

Freeman’s study was funded by the Society for the Study of Addiction.

Police Scotland did not respond to a request for comment.

It is estimated that 7.2% of adults in Europe – 24 million people – used cannabis in the past year.

Across the world, 192 million people use the drug in a variety of markets ranging from heavily sanctioned prohibition to commercialised legal sale.

Recreational use is legalised in Canada and several US states, with medical use permitted in many more countries.

Earlier this year Home Secretary Sajid Javid announced a law change to allow the use of cannabis-related medicinal products amidst fierce campaigning by the families of children with a range of health problems.

Five-year-old Murray Gray, from Edinburgh, was the first child in Scotland to receive medical cannabis for the treatment of epilepsy.

The Scottish Government has called for both the regulation of medicines and drug laws to be devolved.