The Oslo City Court will order two new psychiatric experts to assess the mental health of the 32-year-old right-wing extremist, said the Verdens Gang newspaper on its website on Thursday.
A press conference will be held at 1pm (1200 GMT), it said.
Behring Breivik has confessed to the July 22 twin attacks and claimed he was waging a war against the "Muslim invasion" of Europe.
He first set off a car bomb outside government buildings in Oslo, killing eight people. He then went to Utøya, some 40 kilometres north-west of Oslo, where, disguised as a police officer, he methodically shot and killed another 69 people attending a summer camp, most of them teenagers.
On the same day, he published on the Internet a lengthy manifesto in which he expounded his Islamophobic and anti-multicultural views.
In late November, two court-appointed psychiatrists concluded that Behring Breivik was criminally insane and therefore not accountable for his actions.
The finding meant Behring Breivik would likely be sentenced to psychiatric care in a closed ward instead of going to jail.
Lawyers for some plaintiffs called for a new psychiatric evaluation, questioning the diagnosis of paranoid schizophrenia.
They noted that three psychologists and a psychiatrist who have been monitoring Behring Breivik in prison have, according to media reports, seen no signs of paranoid schizophrenia nor suicidal tendencies.
There are some 700 to 800 plaintiffs in the case, ranging from survivors and families of the victims to owners of businesses or buildings that suffered material damage in the Oslo blast.
The gunman is currently being held under provisional detention while awaiting the trial which opens April 16th.