Breivik, 33, is undergoing a second court-ordered psychiatric evaluation after one last year found him criminally insane, drawing protests from victims' families, The Daily Telegraph reported Friday.
Norwegian state prosecutor Tor-Aksel Busch wrote based on the current situation, he is prepared to write in his instructions to the district prosecutor handling the case that Breivik would be tried as someone who is considered insane.
"The way the case appears at the time, the charges are being brought, there is no basis to request a regular prison penalty," Busch wrote.
But Busch said the prosecution could request during trial up to 21 years in prison.
Breivik is under 24-hour surveillance by a team of psychiatrists, who are to provide an opinion on whether he is suffering from paranoid schizophrenia. A lawyer representing victims' families successfully challenged that assessment, which was based on 36 hours of interviews with Breivik and two psychiatrists.
Several prominent psychiatrists argued it would have been unlikely for a paranoid schizophrenic to have been able to carry out such a carefully planned attack.
Breivik set off a car bomb July 22 outside the government's headquarters in Oslo, killing eight people, then, dressed as a police officer, went to Utoya Island, outside the capital, where he opened fire on a Labor Party youth camp, killing 69 others.
He has said he carried out the attacks to make Norway aware of the threat of militant Islam.
The psychiatric assessments will not be binding on the judge in the trial, scheduled to begin April 16.