In an open letter sent a Norwegian and Swedish media, Breivik complained that since the second of September, he had been confined in isolation in a single cell, which he was only allowed to leave for one hour a day.
He also complained that he was being given less time with prison staff, and that communication was now limited to a small gap in the door.
"Unless the 02/09/15 escalation [sic] is reversed, I will eventually continue the hunger strike until death. I can not stand any more," he wrote.
Breivik is currently serving a 21-year jail sentence at Skien prison for a brutal twin terror attack in 2011, in which he detonated a bomb in the government quarters in Oslo, killing eight people, and then opened fire at a political youth camp, killing 69 more.
In his letter he complained that his conditions meant he could no longer study his political science course at University of Oslo.
"Studying and corresponding is not humanly possible under such circumstances, and this applies to anyone who is isolated under such conditions," he wrote in the letter.
"The decision about the drastic deterioration of prison conditions forced me to drop out of my studies, which in turn means that I will lose my place at the University. The studies, which were made possible for only thirteen full days before the Minister of Justice put an end to them, were the only thing I had."
Breivik's lawyer Øystein Storrvik, who is suing the Norwegian state for human rights violations on behalf of his client, confirms that Breivik's prison conditions have deteriorated.
"I can confirm that there is less of the little that was," told Norway's Dagbladet. "Total isolation from other people has been maintained, while his movements are confined to a smaller space. He also has less time with officers."
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Storvik also confirms that Breivik is preparing to go on hunger strike.
"He writes that he is going on hunger strike until death. He writes that he can not bear anymore."