The letter, originally written on September 29, was sent out by prison authorities on Friday to the Wall Street Journal and the Council of Europe's Committee for the Prevention of Torture, among other organisations.
In the letter, Breivik, who was convicted in 2012 for twin terror attacks which claimed 77 lives, also said in the letter that he was being censored and did not have enough fresh air, showers or exercise.
Breivik's lawyer Todd Jordet confirmed that Breivik had written the letter.
Prison authorities spent far too long considering whatever they could before [the letter] could be sent out," he said.
In the letter received by the Wall Street Journal, Breivik reportedly boasted that if he wanted to retaliate for his poor treatment, he knew how to build weapons out of items he was allowed in prison, such as his Sony PlayStation 2, long screws and plain sheets of paper.
However he claimed that the violent phase of his struggle against Islam and cultural Marxism ended in 2011.
The killer also claims to have written a book more than 1000 pages long, which he claims he is unable to send to a publisher, because he is being blocked by prison authorities.
In a letter written to VG newspaper, the killer claims that he tried to send the book, called The Breivik Diaries, to a publisher on 11 July 2013, and send a 35 page excerpt to VG newspaper, which never arrived.
Breivik's lawyer Todd Jordet confirmed that Breivik's book was nearly complete.
"He has a very detailed draft that will provide the basis for more than 1000 pages of a finished book," Jordet said. "Breivik has even received a commission from a publisher."
According to Jordet the book provides new details on the planning of the attacks, and the reasons for the choices Breivik made.
In a letter received by the German newspaper Die Welt , Breivik expressed doubt that he would ever escape prison.
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"I will not survive this sentence," he wrote.
Karl Gustav Knudsen, the director of Skien prison, where Breivik is currently being held, denied censoring his writings.
"There is freedom of the press in this country, and as long as he does not encourage criminal offences, we will not prevent him from sending out his text," he told VG.
Breivik, who is serving a 21-year sentence, was moved from Ila prison outside Oslo to Skien last year.