Yoga originates from India and dates back some 5,000 years

High-security Ringerike jail near Oslo offered the classes to eight inmates on a trial basis earlier this year.

Prison warden Sigbjoern Hagen said some of the inmates became more irritable and agitated and had trouble sleeping.

He said the prison did not have the resources to treat emotions unleashed by the deep breathing exercises.

The yoga group expressed surprise at the prison's findings.

It said the project had been tested successfully on some 100,000 prisoners in around 15 countries, the AFP news agency reported.

"The reactions we received from the prisoners who participated in the classes were very varied, ranging from completely positive to completely negative," Mr Hagen reportedly wrote in a letter to the group.

On the negative side, the yoga had provoked "strong reactions: agitation, aggression, irritability, trouble sleeping and mental confusion", he said.

The deep breathing exercises are an essential element of Yoga, which originated in India more than 5,000 years ago and aims to harmonise mind, body and spirit.

But such exercises could make inmates more dangerous by unblocking their psychological barriers, Mr Hagen was quoted as saying.