An independent panel investigating how Norwegian authorities dealt with the twin attacks by Anders Behring Breivik that killed 77 people on July 22 last year has said that the Oslo blast could have been prevented and Breivik arrested sooner.

Breivik set off a car bomb outside government buildings in the Norwegian capital, killing eight people, before taking a ferry to Utoeya island, northwest of the capital, where he spent more than an hour gunning down another 69 people, mostly teenagers, and wounding others.

"The attack on the government complex on July 22 could have been prevented through effective implementation of already adopted security measures," the commission said in a report submitted to Prime Minister Jens Stoltenberg.

"The perpetrator could have been stopped earlier on 22 July."

"The authorities' ability to protect the people on Utoeya island failed. A more rapid police operation was a realistic possibility."

The Norwegian police have been harshly criticised for its slow response to the tragedy: more than three hours passed between the Oslo bombing and Breivik's arrest on Utoeya, even though his name was already known to Norway's security services.

The Utoeya shooting lasted about an hour and fifteen minutes, as police struggled to find transport to the small island located on a lake, just 600 metres from shore.

The victims, the youngest of whom had just celebrated her 14th birthday, had been attending a summer camp hosted by the governing Labour Party's youth organisation.