Prison chiefs are scrambling to catch up with inmates who are smoking a form of synthetic cannabis which is almost undetectable by testing equipment or drug hounds.

'Spice' and another synthetic cannabis known as 'Black Mamba' are distinct enough from natural marijuana to be a 'legal high'. They are also invisible to conventional detection tools and sniffer dogs.

The BBC reported Spice is also cheaper than traditional weed, making it a drug of choice for inmates inside prisons.

Spice usage is so widespread that the authorities are taking action by shelling out for updated equipment and also training dogs to recognise the drug.

The rise in popularity of so-called 'legal high' synthetic drugs comes are seizures increase of narcotics in jails across Britain. Meanwhile, the number of prisoners failing random drug tests has fallen.

There were nearly 4,500 drug seizures in prisons in England and Wales in 2014, compared to 3,700 three years ago. Inspections of cells led to 1,300 busts of authentic cannabis and cannabis plants.

A privately-run prison in Doncaster recorded the highest number of drug seizures overall.

A Ministry of Justice spokeswoman said: "It is simply illogical to say that our drugs strategy is failing because of the number of seizures.

"What this in fact shows is that our robust security measures, which include the use of intelligence-led searches and specially trained drugs dogs, are working.

"Our random drug tests show that the numbers testing positive have fallen dramatically over the past 15 years. While this is good progress, we remain just as focused as we always have been on cracking down on drugs in prison."

"It is simply illogical to say that our drugs strategy is failing because of the number of seizures.

"What this in fact shows is that our robust security measures, which include the use of intelligence-led searches and specially trained drugs dogs, are working.

"Our random drug tests show that the numbers testing positive have fallen dramatically over the past 15 years. While this is good progress, we remain just as focused as we always have been on cracking down on drugs in prison."