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London - Prisoners are sneaking drugs into jails by having fake legal letters soaked with liquid ‘zombie drugs’ such as Spice.

The smuggling relies on prison officers not checking the letters because they have legal privilege – meaning they should not normally be opened.

Inmates at HMP Deerbolt in County Durham are getting friends on the outside to spray liquid psychoactive substances on letters and photos, which they can then roll up and smoke or sell to other prisoners.

HM Inspectorate of Prisons said all personal mail to convicts was now being photocopied before being delivered to cells to tackle the problem.

Its report said: ‘Prison staff phoned solicitors who were sending privileged mail to a prisoner to check it was genuine. Three letters had been opened when it had been confirmed they were not genuine.’

Restrictions had been placed on the letters, books and clothing inmates could take into the prison to reduce smuggling, it added. Nearly one in six prisoners have said they had developed a drug habit behind bars.

The prison, which holds 400 men aged 18 to 24, has also bought its own drone to keep an eye on inmates.