Norway's police apologised on Thursday for not acting faster to stop the massacre of 69 people at a Labour Party youth camp last summer by an anti-Islamic gunman who had earlier set off a bomb in central Oslo that killed eight.

The violence, Norway's worst since World War Two, profoundly shocked the usually peaceful nation of 4.8 million and victims' families have strongly criticised the police for their slow response.

They have complained that too many police officers were on holiday and that no helicopter was ready to dispatch.

On behalf of the Norwegian police I would like to apologise that we did not succeed in apprehending the attacker earlier, Police Director Oeystein Maeland told a news conference.

He said a collapse in communication and a police boat so overloaded with officers that it took on water were key factors that delayed the response to the island summer camp.

Anders Behring Breivik, 33, has admitted setting off a bomb at an Oslo government building before going on a shooting spree at Utoeya island 40 kilometres (25 miles) away, to punish traitors with immigrant-friendly attitudes.

A court-ordered psychiatric team has diagnosed Breivik as psychotic, and a second examination is under way. His trial on terrorism and murder charges is due to begin on April 16.

It took Oslo's special police tactical team about an hour to travel by car and boat to camp of the ruling Labour Party's youth wing after the first shots were reported there. Breivik then surrendered willingly.

Every minute was a minute too long, Maeland said after submitting a report in which the national police directorate listed dozens of ways to improve the response to future attacks.

It is a heavy burden to know that lives could have been spared if the assailant had been stopped earlier, Maeland added.

The chief of the police district where the massacre occurred, Sissel Hammer, said police theoretically could have shortened Breivik's attack by 16 minutes if the response had been perfect. How many lives could have saved could not be determined, Maeland said.

Norway's domestic security service, a separate body, was set to present its own self-critique on Friday and would likely concede weaknesses in information gathering and analysis, the Aftenposten newspaper reported.

As well as the police report, a parliamentary committee and an independent commission appointed by the government are also evaluating the police response.

(Reporting by Walter Gibbs; Editing by Karolina Tagaris)