Anders Behring Breivik, who carried out the July 22 attacks that killed 77 people in Norway, may never go to jail after psychiatrists ruled him criminally insane, a prosecutor said Tuesday.

Instead, the 32-year-old gunman could spend the rest of his life in a mental institution, prosecutor Inga Bejer Engh told reporters in Oslo.

The prosecutor was speaking after forensic experts submitted a psychiatric evaluation Tuesday in which they found the gunman "insane".

"In such instances that the person suffers from such a serious disorder that it would not be warranted to sentence him to prison... he can be ordered to stay in mental health care institutions," said Bejer Engh.

The 243-page psychiatric report found that Behring Breivik had over time developed "paranoid schizophrenia," with two experts describing a person in his own "delusional universe," another prosecutor, Svein Holden, said.

Holden said the report also concluded that the rightwing extremist had "grandiose illusions whereby he believes he is to determine who is to live and who is to die."

He "committed these executions out of love for his people as he describes it," Holden said.

Behring Breivik predicted a scenario whereby "his organisation, the Knights Templar, take over power in Europe and he puts himself forward tentatively as the future regent in Norway."

Tuesday's psychiatric evaluation will be examined by a committee of forensic experts in order to ensure that it meets professional requirements.

The court will have the final say on whether Behring Breivik can be held responsible for the crimes, though it generally follows the experts' recommendations.

On July 22, Behring Breivik first set off a car bomb outside government buildings in Oslo, killing eight people.

After that, he went to the island of Utoeya, some 40 kilometres (25 miles) northwest of Oslo, where, disguised as a police officer, he spent nearly an hour and a half methodically killing another 69 people, most of them teenagers.

Although he has confessed to the facts, Behring Breivik has refused to plead guilty, claiming he was waging a war and that his actions were "atrocious but necessary."