The question of Breivik's sanity has been a focal point of his 10-week trial, with the defendant not denying his actions while craving recognition of the right-wing extremist ideology that he says prompted the massacre.

The panel of experts was enlisted to assess the quality of two opposing conclusions, one by two court-appointed psychiatrists who last year found Breivik to be psychotic and therefore not  responsible for his actions, and a second court-ordered evaluation that found him to be sane.

The experts approved the findings of the first exam, but found "major deficiencies" in the second opinion even after the two authors provided requested complementary information, according to a highly technical letter published on Friday.

Legal experts said the panel's conclusion was neither a rejection nor an approval of the counter-expertise.

In the panel's letter, it stressed that "the questions put to the psychiatric experts … do not relate to their conclusion itself" but to the foundation they had built their conclusion on.

The opinion can nonetheless be considered weakened compared with the first expert evaluation, which is bad news for Breivik's defence.

Breivik, 33, is intent on proving his sanity to ensure that his ideology — described as a crusade against multiculturalism and a pending "Muslim invasion" of Norway and Europe — not be written off as the rantings of a lunatic.

"We would of course have preferred that the panel had no objections concerning the second expert opinion," Breivik's main lawyer Geir Lippestad told AFP, while stressing that in the end it would be up to the five Oslo district court judges to decide which expert opinion to lend most weight when they come to their verdict next month.

Prosecutor Svein Holden meanwhile reiterated that he wanted to keep all doors open until the end of the trial, leaving open the possibility to request that Breivik be sent either to prison or to a closed psychiatric ward.

On July 22nd 2011, Breivik first bombed a government building in Oslo, killing eight people, before going on a shooting rampage on the nearby island of Utøya where the ruling Labour Party's youth wing was hosting a summer camp.

He killed 69 people in his island massacre, most of them teens, with the youngest having just celebrated her 14th birthday.

If he is found sane, he faces a 21-year jail term which could be extended indefinitely if he is still considered a threat to society. If he is found insane he could receive closed psychiatric care, possibly for life.