An Oslo court verdict last month handing gunman Anders Behring Breivik a 21-year prison sentence stands after he told his lawyers he would not lodge an appeal by Friday's deadline, they said.

Norwegian Breivik was sentenced to his country's maximum prison sentence for his July 22 twin attacks last year that left 77 people dead, though it can be extended indefinitely if he is still considered a threat to society.

The 33-year-old rightwing extremist has until Friday at midnight (2200 GMT) to place an appeal, but he has insisted since the verdict fell on August 24 that he would not do so because that would legitimise the Oslo district court which he says he does not recognise.

"There is no doubt that the July 22 case is over today," chief defence lawyer Geir Lippestad said Friday after visiting his client in prison.

"He maintains ... that he will not appeal," another defence lawyer, Vibeke Hein Baera, added.

On July 22, 2011, Breivik set off a car bomb outside the government offices in Oslo, killing eight people, before going to the island of Utoeya, northwest of the capital, where he spent more than an hour gunning down another 69 people, mostly teenagers, attending a Labour Party youth camp.

He confessed to the attacks, calling them "cruel but necessary" to protect his country from the multiculturalism his victims embraced and which he hates.