The Oslo appeals court ruled that a lower court's decision to order a second examination was legitimate given the criticism and debate over the first evaluation.
If the second opinion confirms the diagnosis drawn by two court-appointed psychiatrists, Anders Behring Brevik, a 32-year-old right-wing extremist, will likely be sentenced to a closed mental ward instead of prison.
Behring Breivik had appealed the Oslo district court's decision to order a new exam, angry over the fact that it had based its decision in part on medical documents that were classified.
Three psychologists and one psychiatrist monitoring Behring Breivik in prison were quoted in the media as saying they saw no signs of the paranoid schizophrenia diagnosed by the two court-appointed psychiatrists.
Those comments sparked a wave of criticism in Norway, where many have voiced concern that Behring Breivik may be found not accountable for his actions, despite the years he spent planning his massacre and his calm demeanour as he executed his attacks.
On July 22nd, the man who has claimed to be on a crusade against multi-culturalism and the "Muslim invasion" of Europe set off a car bomb outside government buildings in Oslo, killing eight people.
He then went to Utøya island, some 40 kilometres north-west of Oslo, and, dressed as a police officer, spent more than an hour methodically shooting and killing another 69 people, mainly teens, attending a summer camp hosted by the ruling Labour Party's youth wing.
On Thursday, the appeals court said the decision to order a second exam was based on an "overall view" and not just the opinions of the prison monitors.
A lawyer for Behring Breivik, Odd Ivar Grøn, told AFP his client may appeal to the Supreme Court.
The two new court-appointed psychiatrists have meanwhile asked to have Behring Breivik committed to a psychiatric hospital for observation.
They are due to present their diagnosis on April 10th, just days before his trial opens on April 16th.
Ultimately it is the court that will decide whether Behring Breivik is criminally insane.