Breivik's attorney Øystein Storrvik, cheered the court's decision to side with his objections to the judge's participation in the appeal case.
“We can confirm that we have succeeded in the petition and the court agreed with our argument,” Storrvik said.
Mo was state secretary in the Ministry of Finance uder former PM Jens Stoltenberg's first cabinet in 2000- 2001, which was a pure Labour government.
Breivik's 2011 rampage on the island of Utøya targetted the summer meeting of Labour's youth party, AUF. The right-wing extremist blamed the party for the rise of multiculturalism in Norway.
“That [Mo's role] was 15 years ago and that it was a short period, might indicate that this relationship cannot be regarded as any particular circumstance that is likely to undermine confidence in her impartiality,” the court wrote in its ruling.
“On the other hand, state secretary is an important and high-profile political role, and she may thus appear outwardly as a supporter of the policy that Breivik attacked. Moreover, there were several ministers and state secretaries in this government who were directly affected by Breivik's attacks on Utøya. In the court's view, this special relationship as a whole can give the outside world and Breivik a reasonable and justifiable reason to doubt her impartiality,” the ruling went on.
Brevik's lawyer filed its complaint over Mo earlier this summer but did not publicly detail the grounds for the complaint.
It was not known immediately known whether someone had been appointed to take over for the removed judge.
The state's appeal against Breivik's partial victory in his complaint about prison conditions is scheduled to be carried for November 29th, and is due to take four days. Storrvik, however, has requested a new date due to a scheduling conflict.
Breivik sued the state of Norway for prison conditions that he says violate his human rights. In April, the Oslo District Court sided with the convicted terrorist's claims that being held in solitary confinement in the Skien prison violates his rights under Article 3 of the European Convention on Human Rights.
However, the court ruled against the extremist on his claim that the strict controls on his correspondence also amounted to a violation of Article 8, which guarantees the right to a private life.
The Office of the Attorney General filed an appeal against the decision on the grounds that it disagreed with both the court's interpretation and the evidence presented in the case.
Breivik is serving a 21-year prison term -- which can be extended indefinitely --for his attacks on Utøya and the Olso government district on July 22nd, 2011, in which 77 people, mostly teenagers, were killed and a large number were seriously injured.