Norwegian mass murderer Anders Breivik, who killed a total of 77 people, most of them teenagers, last July has been legally declared sane ahead of his trial next week.

Breivik, a radical right-wing extremist, committed two attacks on 22 July 2011, first planting a car bomb in Oslo, before slaughtering 69 people in a summer youth camp later in the day.

The 32-year-old was originally declared insane by a psychiatrist following the attacks.

But a new team studied Breivik's behaviour in prison, and found that, despite apparently living in his "own delusional universe where all his thoughts and acts are guided by his delusions", Breivik was neither psychotic nor schizophrenic.

The experts found that Breivik was not in a compromised mental state as he first impersonated a police officer before killing eight with a car bomb, then headed to a small, secluded island camp for young activists, where he opened fire on children, killing 69.

In a letter to Norwegian media, Breivik himself stated he was sane, and called the possibility of being placed in a psychiatric ward "a fate worse than death".

Breivik has pleaded guilty to the murder of his 77 victims, and will go on trial from Monday 16 April.

Breivik would have faced trial next week regardless of his sanity, however the assessment would decide whether he would serve a prison sentence or be committed to a psychiatric hospital.

Anders Breivik To Stand Trial
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The remains of the van used for to hold a bomb left outside a government building by Anders Breivik on the morning of the 22 July.

The bomb killed eight and devastated the nearby area.