"We have examined a good part of the report that details the conversations he had with the psychologists," Odd Ivar Grøn told the daily Verdens Gang.
Anders Behring Breivik, 32, "reacted by saying that it contained factual errors (and) lies and that his statements were taken out of context," Grøn was quoted as saying.
Prosecutors last Tuesday declared the right-wing extremist criminally insane when he killed 77 people on July 22nd after two psychiatrists who examined him concluded that he suffered from paranoid schizophrenia.
The lawyer said he spent six hours discussing the 243-page report with Behring Breivik.
"He thinks they considered some of his statements bizarre that he didn't think were bizarre. And he does not share their judgement that he is mentally ill," Grøn said.
"He is concerned over the fact that these experts do not have enough knowledge of political ideologies," Grøn told the paper.
By removing Behring Breivik's criminal responsibility, the diagnosis will probably see him sentenced to receive psychiatric care in a closed institution -- possibly for the rest of his life -- instead of prison, and has divided opinion in Norway.
The report by psychiatrists Synne Serheim and Torgeir Husby has still to be reviewed by a panel of experts to ensure that it conforms with professional standards.
Behring Breivik's trial is set to open April 16th and is expected to last around 10 weeks.
In the attacks, Behring Breivik first set off a car bomb outside government buildings in Oslo, killing eight people, before going on a shooting rampage on the nearby island of Utøya.
For nearly an hour and a half, he methodically gunned down another 69 people, most of them teenagers attending a summer camp of the ruling Labour Party youth organisation.
Behring Breivik, an Islamaphobe, accused the party of promoting the multicultural society he despises.