Norway violated mass killer Anders Behring Breivik's human rights by keeping him in a "completely locked world" after being sentenced for killing 77 people in twin attacks in 2011, a court has ruled.
Wednesday's ruling, which took many by surprise, found that the killer had been subjected to strip searches, had been woken up hourly by guards for long periods and that the authorities had done little to alleviate the impact of his isolation.
Breivik killed eight people in a bomb attack in Oslo in July 2011 before attacking a youth meeting of the Labour Party on an island to the northwest of the capital, killing 69 people.
He took Norwegian authorities to court in March, accusing them of exposing him to inhuman, degrading treatment or punishment in breach of the European Convention on Human Rights.
Breivik protested his isolation from other inmates and from outsiders who are not professionals.
"The prohibition of inhuman and degrading treatment represents a fundamental value in a democratic society. This applies no matter what - also in the treatment of terrorists and killers," judge Helen Andenaes Sekulic said in her ruling.
The verdict said the Norwegian state had broken Article 3 of the convention, pointing to the fact that Breivik is spending 22 to 23 hours a day alone in his cell.
The ruling, however, said the Norwegian state had not violated Breivik's right to a private and family life.
It said strict censorship of his letters was "in line with the law".
Breivik wants to exchange letters with outsiders, including several far-right extremists.
The state must pay Breivik's legal fees of some 331,000 Norwegian crowns ($A52,090), the judge ruled.
Breivik's lawyer Oeystein Storrvik declined to say what Breivik's reaction was to the ruling but his client would not appeal the part of the verdict that ruled against him..
Lawyers representing the state said they would consider whether to appeal.
The justice minister, Anders Anundsen, whose ministry was being sued by Breivik, did not say whether the verdict would be appealed.
