Norwegian mass killer Anders Behring Breivik, who murdered 77 people in 2011, has officially changed his name to Fjotolf Hansen, his lawyer said today.

Oeystein Storrvik declined to give Breivik's reasons for adopting Hansen, one of Norway's most common surnames, or the extremely rare Fjotolf.

"He told me some reasons but I don't want to talk about what he told me," Storrvik told Reuters, confirming Norwegian media reports of the name change.

In Norway, citizens can freely change their names in the official register but are not allowed to pick words that are likely to cause offence to others or harm the individual. It was unclear when he made the change.

On Thursday, the Norwegian Supreme Court said it would not consider an appeal lodged by Breivik protesting against his prison conditions.

The anti-Muslim far-right extremist killed 77 people in Norway's worst peacetime atrocity in July 2011. He killed eight with a bomb in Oslo and then gunned down 69, many of them teenagers, at a youth meeting of the then-ruling Labour Party.

Norway's Statistics Bureau says that there are more than 52,000 people with the surname Hansen in a population of five million. On Fjotolf, it merely says that it is used by fewer than four people.