Former inmates are deliberately getting sent back to prison to cash in on lucrative profits on offer for selling drugs previously known as "legal highs", according to a new report.

Prices for the substances can jump 33-fold once they cross into jails - providing prisoners with an incentive to go back behind bars to make money, researchers claimed.

A gram of synthetic cannabinoids, which mimic the effects of cannabis, can cost £3 on the outside but can fetch up to £100 when sold in prisons, the study said.

It claimed to have uncovered strong evidence that the licence recall system - under which offenders can be brought back to custody - was "routinely and systematically" abused to bring the drugs into prisons.

The paper, based on research conducted in an adult male prison in England, suggested that recently freed inmates committed minor infractions, such as missing probation meetings, in order to return to jail.

Prisoners reported being able to make £3,000 in four weeks by bringing in an ounce (28 grams) of synthetic cannabinoids.

One prisoner even claimed that another inmate had made £100,000 dealing the substances during a six-month sentence - although the report acknowledged that stated profits could be prone to exaggeration.

It detailed a number of "novel" reported smuggling methods, including via drones or sprayed onto books, letters and children's drawings.

Lead researcher Dr Rob Ralphs, senior lecturer in criminology at Manchester Metropolitan University, said: "It is no exaggeration to say that the synthetic cannabinoid market has exploded and unleashed a series of devastating impacts on prisons, prisoners and prison staff.

"Traditional drugs have almost been wiped out and replaced with these extremely powerful synthetic cannabinoids because prisoners are attracted by high profit margins and their lack of detection in drug tests.

"Our research found that prisoners' motivation for taking synthetic cannabinoids was to escape the boredom of prison life and to avoid positive drug tests but their impact is extremely serious."

The study - published in the International Journal of Drug Policy - comes at a time when the state of jails in England and Wales is under intense scrutiny.

New psychoactive substances, which were commonly referred to as legal highs before they were made the subject of a blanket ban earlier this year, have been identified as a "game-changer" as prisons are hit by surging levels of violence.

The government is pursuing a number of measures to tackle the problem under its prison reform plans, including "no fly zones" to stop drones dropping contraband into jails and mandatory drug testing across the estate.

Criminally tasty: Britain's first prison restaurant Show all 4 1 /4 Criminally tasty: Britain's first prison restaurant Criminally tasty: Britain's first prison restaurant Serving thyme: chef Alberto Crisci (centre) with his staff of convicts DAVID SANDISON Criminally tasty: Britain's first prison restaurant A prisoner prepares food which will be served to staff, visitors and commercial customers DAVID SANDISON Criminally tasty: Britain's first prison restaurant The Clink operates within HMP High Down. The Idea was conceived by chef Alberto Crisci PA Criminally tasty: Britain's first prison restaurant Nick Harding is served by Phillip inside H.M. Prison High Down DAVID SANDISON

More than 300 drug detection dogs have also been trained to identify psychoactive substances concealed in parcels and on people.