Spice is so rife in Britain’s jails that prisoners are now twice as likely to be addicted to the drug when they leave than before they enter, a leading expert has warned.

Dr George Ryan, of Public Health England, said spiralling use of the drug – a potent form of synthetic cannabis - was behind an explosion of violence in prisons, causing ‘deaths, bullying and violence’.

Dr Ryan, a Government advisor, warned that it was relatively easy for prisoners to smuggle in spice as it was a liquid which could be sprayed onto regular tobacco – which most prisons allow. It can even be sprayed onto a piece of paper ‘the size of a thumbnail’, to be smoked later.

He revealed that urine tests carried out in 10 prisons in north west England had showed around 8 per cent of prisoners tested positive for spice on arrival but 16 per cent were positive on release.

By contrast, levels of other drugs like cannabis, cocaine and heroin all dropped dramatically during prison sentences. Just one in 100 prisoners tested positive for cocaine on release compared to one in four on arrival.

Speaking at PHE’s conference at Warwick University, Dr Ryan said: ‘Perhaps the most alarming statistic of all is that prisoners are twice as likely to use spice when they leave prisons as when they arrive.

‘So, effectively, use of spice doubles when people are incarcerated.  ‘It’s a very potent drug so people get a lot more bangs for their buck. It remains a very affordable drug in prison for some people. Higher potency forms increase the risk of people becoming dependent.’

Spice is not one single drug but the name for a group of similar chemicals known as synthetic cannabinoids designed to mimic the effects of cannabis. The drugs were sold as legal highs but were banned last year.

Experts say newer versions are stronger and more unpredictable. The highly addictive drug can leave users in a ‘zombie-like’ state or trigger psychotic episodes.

Spice is attractive to prisoners because it is cheap and was previously hard to detect – effective tests have only recently been developed. Until May last year there were no sanctions for prisoners possessing spice in jail, but the law now says offenders caught with the drug could face up to two further years in custody and a fine.

But prison officers warn use of the drug is at epidemic levels. In July, a two-day riot linked to a jump in supplies of the drug took place at The Mount prison in Hertfordshire.

Home Office figures show two thirds of all prison drugs seizures are for so-called ‘new psychoactive substances’, of which 99 per cent are spice.

In the first 10 months of 2015, officers at HMP Forest Bank, near Manchester, seized 4.4kg of spice – 39 times more than the 114g of cannabis and 210 times more than the 21g of heroin seized over the same period.

Dr Ryan said the wide use of spice could explain an increase in violence in prisons, as potent strains could leave up to 10 or 12 prisoners needing hospital treatment in a day. He said: ‘You have the toxic combination of wide-ranging effects and high, variable and unpredictable potency. In a closed environment like a prison it’s particularly challenging.

‘Each [hospitalised] prisoner is accompanied by a minimum of two people as an escort - that would lead to a meltdown with already over-worked staff brought to breaking point.

‘Other prisoners who have done nothing wrong will be confined to their cells due to staff shortages and will be rightly frustrated, aggrieved and angry… This level of dissatisfaction will lead to prisoners being volatile and probably is the explanation for some of the disorder we are seeing in our prisons over the past 12 month or so.