A republican prisoner in Lithuania is being held in "inhuman and degrading conditions" that are in clear breach of the European Convention on Human Rights, his lawyer has said.

Michael Campbell from Dundalk has been prevented from seeing any family member for almost three years. He is facing charges of attempting to buy arms for the Real IRA which he is denying.

Campbell (37) was arrested in January 2008 as part of a sting involving MI5, gardaí and Lithuanian police. His lawyer, Peter Corrigan, recently visited him in Lukiskes prison in Vilnius.

Corrigan said: "I was horrified. He shares a tiny cell with six other prisoners. The toilet they all use is a hole in the ground of the cell. My client is locked up 23 hours a day. There is no exercise yard. He is just allowed a brief walk outside his cell every day.

"Since his arrest, Mr Campbell has been prevented from seeing a single family member or friend – that is unheard of in any prison in the world. It was 18 months before he was even allowed to make a phone call home. His cell mates don't speak English, so he has no communication with other human beings which is taking its toll psychologically."

Corrigan said he also had "serious concerns" about his client's physical health: "He has lost a lot of weight. He gets three meals a day which all consist of raw fish. The conditions in any Irish jail... don't compare to this. I've visited Belmarsh prison in England where the cells are like concrete coffins but it's a holiday camp compared to Lukiskes."

Human rights group British Irish Rights Watch is monitoring the situation. A report for the European Committee for the Prevention of Torture uncovered atrocious conditions in the jail.

Campbell, who was in Vilnius buying counterfeit cigarettes, was subject to a major sting when a secret agent posing as a cigarette dealer allegedly offered to show him arms. If convicted, he faces up to 20 years in jail. His trial has been adjourned until later this month.