Breivik, a right-wing extremist who killed 77 people in a bombing and shooting rampage in July 2011, is being held in a high-security unit in southeast Norway's Skien prison with severely limited contact with the outside
The 36-year-old, who carried out the attack in opposition to Norway's multiculturalism, has likened his prison conditions to "torture" and is suing the Norwegian state. Court hearings are due to begin in March.
Breivik has also repeatedly threatened to go on a hunger strike.
"The regimen in the very high security unit imposes very strict conditions on inmates' freedom of movement and their possibility to have contact with other people," ombudsman Aage Thor Falkanger, tasked with investigating injustice by public agencies, wrote after visiting Breivik's prison.
"This, and the fact that in reality there is an extremely limited number of inmates in the very high security unit, means that this regimen represents an elevated risk of inhumane treatment," he added in his report.
The report called for more interaction between guards and inmates to "reduce the risk of damage" linked to solitary confinement, as well as a review on the adoption of "less intrusive security measures than handcuffs".
According to public broadcaster NRK, prison authorities have already adopted some measures to ease Breivik's isolation, such as allowing him to engage in activities with guards for one hour a week.
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On July 22, 2011, Breivik killed eight people in a bomb attack outside a government building in the capital Oslo and later murdered another 69 people -- most of them teenagers -- when he opened fire at a Labour Youth camp on the island of Utøya.
In August 2012, he was handed a 21-year prison sentence, which can be extended if he is still considered a danger to society.