Lawyer Geir Lippestad said the best weapon against Breivik's anti-Islamic and anti-multicultural stance was open debate.
"What is most worrying is that I keep receiving messages from around the world that Breivik is increasingly a cult figure in some circles," Lippestad wrote in his memoirs published in Norway on Thursday.
"When you see images from Russia, the United States, Britain, Germany, Greece, Sweden and other countries where he has supporters, Breivik is undoubtedly becoming a model who can influence other people to have thoughts, ideas and plans that could end up transforming young people into terrorists and not into law-abiding citizens," he wrote.
Breivik is serving a 21-year jail sentence for killing 77 people in Norway in twin attacks on July 22, 2011 that he described as "cruel but necessary".
He killed 69 people, mostly teenagers, in a gun rampage at a Labour party youth camp after setting off a massive bomb outside the main government building in Oslo, killing eight.
Breivik had laid out his Islamophobic and anti-multicultural ideology in a rambling 1,500-page online manifesto posted online shortly before the attacks.
"Upon reflection, it's not as inexplicable as it seems," Lippestad said.
"There are a lot of young people out there who are searching (for themselves). There are a lot of people out there who are angry, who feel a sense of hopelessness over their situation and who lack the basic education or the proper tools… that would normally make them sceptical to violence and totalitarian thinking," he wrote.
In his book, Lippestad said the best way to combat extremism was to have an open exchange of ideas.
Lippestad has been broadly hailed in Norway for the way in which he defended Breivik, where he argued in the name of the rule of law while keeping his distance from his client's deeds.
Breivik, held in isolation in a high security prison near Oslo, has complained several times of censorship in his communication with the outside world since all of his mail must be screened by prison officials.
The fact that Breivik is still allowed to communicate with extremist circles is "unfortunate but, alas, something we must tolerate in the name of freedom of expression," Lippestad said.
"The solution is not to ban, but to cultivate the counter-arguments that are so forceful that the extremist ideas are eroded and reduced to ashes."