Anders Behring Breivik has threatened to starve himself to death in protest at his treatment in a Norway prison, according to media reports.

The mass killer claims to have been kept in isolation since September 2, with time outside his cell limited to an hour a day.

Anders Breivik, now 36, killed 77 people in 2011 when he bombed central Oslo before going on a shooting spree at a youth camp on Utoya island.

He was sentenced to 21 years in 2012.

Anders Breivik’s claims about deteriorating prison conditions were made in a letter to media outlets in Norway and Sweden.

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In his letter, Anders Breivik says harsh prison conditions have forced him to drop out of a political science course at the 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 according to English news site The Local.

Anders Breivik said that if conditions remained unchanged he would continue the hunger strike until he died, Norwegian media reported.

Norwegian Justice Minister Anders Anundsen declined to comment on Anders Breivik’s claims, according to the Dagbladet newspaper.

The University of Oslo admitted Anders Breivik as a full student in July 2015, explaining that inmates had a right to higher education if they won entry to courses.

Anders Breivik has previously complained of “inhumane” prison conditions, including that his coffee was being served cold.