A sociologist called as a witness by the defence at the trial of Anders Behring-Breivik says that the self-confessed killer "cannot distinguish" reality and computer games.

Professor at Oslo university Thomas Hylland Eriksen, told reporters that Breivik is a fantasist who created an alternative persona for himself through online computer games which he sometimes brought into the real world.

"I think one factor which hasn't been taken sufficiently into account is his obsession with a certain kind of computer game and his way of using the internet in order to create for himself a kind of alternative reality.

"He does not seem to be very successful in distinguishing between the virtual reality of World of Warcraft and other computer games, and reality," said Eriksen.

He went on: "So, when he puts on his uniform he is no longer the lone, slightly-unsuccessful young man from the west end of Oslo, who never completed an education, never did really well in his working life.

"He becomes a knight, a defender of the civilisation of Europe against the invading Muslims."

Breivik, citing his involvement in a group called the Knights Templar, chafed in court on Wednesday following claims that the organisation, though it existed in name, was mostly fictitious.

The original Knights Templar were a medieval brotherhood of knights that prosecuted and financed anti-Islamic crusades.