Intelligence services could have learned about Breivik's plans months before the attack made him the worst mass killer in Norway's peacetime history, the commission's report said.

The government building he bombed should have been better protected and he should have been stopped before he gunned down dozens of victims, mostly teenagers, on an island as police struggled to find a working helicopter and a suitable boat.

"All in all, July 22 revealed serious shortfalls in society's emergency preparedness and ability to avert threats," the commission said. "The challenges turned out to be ascribable to leadership and communication to a far greater extent than to the lack of response personnel," it said.

Breivik first detonated a fertiliser car bomb outside government headquarters in Oslo, killing eight people, then travelled to the ruling Labour Party's summer camp on Utoeya island where he gunned down 69 victims unimpeded.

Authorities become aware of his suspicious activities months before when he bought items that could be used to make bombs but intelligence service failures meant he was not put on a watch list, the commission said in the 482-page report.

Breivik admits the attacks but denies criminal guilt, claiming to be a political activist who attacked the ruling party for its support of Muslim immigration, which he says has adulterated pure Norwegian blood.

His 10-week trial ended in June and a court is expected to deliver its verdict on August 24, with prosecutors asking the five judges to declare Breivik insane. If deemed insane, he faces indefinite mental care in a facility inside a maximum security prison while if ruled sane, he faces a 21-year prison sentence with the possibility of indefinite extensions.