Image caption Juan Mendez said the majority of countries abused the practice of solitary confinement

The UN's lead investigator on torture has called for governments to end the use of long spells of solitary confinement in prison.

Juan Mendez said such isolation could cause serious mental and physical damage and amount to torture.

He said it should not be used on people with mental disabilities or juveniles.

Mr Mendez said short term isolation was permissible for prisoner protection but all solitary confinement longer than 15 days should be banned.

He told a UN General Assembly human rights committee that solitary confinement as practised in a majority of countries was "subject to widespread abuse".

Mr Mendez, a professor of law at American University in Washington, cited studies indicating harmful physical and mental effects after just a few days of solitary confinement.

"Considering the severe mental pain or suffering solitary confinement may cause, it can amount to torture or cruel, inhuman or degrading treatment or punishment when used as a punishment, during pre-trial detention, indefinitely or for a prolonged period, for persons with mental disabilities or juveniles," he said.

"Segregation, isolation, separation, cellular, lockdown, supermax, the hole, secure housing unit... whatever the name, solitary confinement should be banned by states as a punishment or extortion technique."

He said it was estimated that in the US, 20-25,000 prisoners were being held in isolation.

Mr Mendez also criticised Chinese authorities for keeping a woman in isolation for two years out of an eight-year sentence for supplying state secrets to foreigners.