Chief judge Wenche Elizabeth Arntzen called a 30-minute recess shortly after the opening of the second day of the trial to determine whether lay judge Thomas Indrebø was unfit because of comments he posted on a website the day after Breivik's July 22nd twin attacks that left 77 people dead.
"The death penalty is the only fair outcome in this case!!!!," Indrebø wrote on July 23rd, according to a report published in anti-racist magazine Vepsen on Tuesday.
"The lay judge himself has acknowledged that he made these comments on July 23rd," Arntzen told the court.
Substitute judges are present, so if Indrebø is deemed unfit, he will likely be immediately replaced and the trial should continue as scheduled.
The case is being heard by two professional judges and three lay judges.
The death penalty does not exist in Norway.
Breivik risks either a 21-year jail term, which could then be extended indefinitely if he is still considered a threat to society, or closed psychiatric care, possibly for life.
Tuesday's proceedings had been due to focus on Breivik's own testimony.
After sitting through the first day of the trial Monday listening to the prosecution present a devastating case against him, Breivik will have five days to explain his massacre, although many fear he will attempt to take the opportunity to spread his Islamophobic ideology.
On Monday, the 33-year-old entered his plea of "not guilty" and voiced his disdain for the court.
"I acknowledge the acts, but not criminal guilt, and I claim legitimate defence," said Breivik, who is accused of "acts of terror," also telling the judges he did not "recognise the Norwegian court."
Tuesday morning, as at the opening of the trial, he extended his clenched right fist in a right-wing salute as his handcuffs were removed on entering the courtroom.
Breivik's defence team have insisted their client will not use the court to launch into an ideological rant on Tuesday, but asked the court's permission for him to be permitted to read a prepared text, which would take about 30 minutes.
The court said Tuesday he would be allowed to make his statement but television and radio would not be allowed to rebroadcast his words.
During his testimony, Breivik "will not only defend (his actions) but will also lament, I think, not going further," Breivik's main defence lawyer Geir Lippestad warned last week.
That lack of compassion for his victims and their families was evident Monday: Breivik sat stony-faced for almost an hour as prosecutors read aloud a long list of names of the dead and injured and recalled chilling details of his massacre.
On July 22nd, Breivik killed eight people when he set off a bomb in a van parked at the foot of buildings housing the offices of Labour Prime Minister Jens Stoltenberg, who was not present at the time.
He then travelled to Utøya island where, dressed as a police officer, he spent more than an hour methodically shooting at hundreds of people attending a ruling Labour Party youth summer camp.
The shooting spree claimed the lives of 69 people, trapped on the small heart-shaped island surrounded by icy waters. It was the deadliest massacre ever committed by a lone gunman.
Breivik showed no emotion Monday as the prosecution presented graphic surveillance footage of his Oslo bombing and a desperate emergency call from a young woman hiding in a bathroom as he went on his shooting rampage on Utøya island.
He did however tear up as the court viewed a 12-minute anti-Islam film he made summarizing his manifesto.
Lippestad told reporters after the first trial day that his client's tears appeared to be linked to his feelings that his attacks were "cruel but necessary ... to save Europe from an ongoing war."
Some of the survivors and victims' relatives also said they did not interpret the tears as remorse.
"I personally feel that him crying was basically him being moved by what he had accomplished. It was not a sign of regret at all," John Kyrre Lars Hestnes of the July 22 Support Group told AFP.