A Norwegian man who bombed his country's capital before gunning down teenagers at a summer camp for the ruling Labour Party's youth wing last July has been declared criminally insane.

A prosecutor said on Tuesday that court-appointed psychiatrists had found Anders Behring Breivik mentally unsound,  meaning he may be committed to a psychiatric institution indefinitely rather than sent to jail.

Breivik's bomb attack in Oslo and the shooting in Utoeya left at least 77 people dead in Norway's worst violence since World War II.

"The conclusion is ... that he is insane," Svein Holden, the prosecutor, 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 suspect could be held as long as he poses a threat to society and may be later 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, the Reuters news agency said.

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.