Court-appointed psychiatrists have concluded that Norwegian mass killer Anders Behring Breivik is criminally insane, a prosecutor said on Tuesday, meaning he may be committed to a psychiatric institution indefinitely rather than sent to jail.

Breivik killed 77 people in July by bombing central Oslo and then gunning down dozens of mostly teenagers at a summer camp for the ruling Labour Party's youth wing.

"The conclusion is ... is that he is insane," prosecutor Svein Holden told a news conference. "He lives in his own delusional universe and his thoughts and acts are governed by this universe."

If the court accepts the psychiatrists' conclusions, the anti-immigration militant could be held as long as he poses a threat to society and may be released if found to be healthy.

Norwegian courts can challenge psychiatric evaluations or order new tests but it is rare for them to reject such a professional opinion.

If the evaluation is upheld, Breivik would not be put on criminal trial but would face a court hearing to rule on his criminal insanity and the length of his commitment to a psychiatric institution.

Breivik could then face similar hearings periodically to determine if he needs to remain committed, and could be held for life if he remained a threat.

Holden said Breivik had developed paranoid schizophrenia and was psychotic at the time of the attacks, and that his condition was persisting.