Breivik, a 34-year-old right-wing extremist who is serving a 21-year jail sentence for killing 77 people in twin attacks in July 2011, is being held under strict prison conditions.

Officials at the high-security Ila prison near Oslo decided to extend his strict security regimen when it came up for review Tuesday, and denied his request to attend the funeral, lawyer Tord Jordet told AFP in an email.

Wenche Behring Breivik, who died age 66, had shunned the spotlight after her son's attacks. She appeared to be the person closest to him, and he had described her as his "Achilles heel".

A single mom, she struggled to raise her son and his half-sister alone, and social services very early on hinted that Breivik might not have been receiving proper care at home. But he was never removed from her care.

As an adult, he returned to live with her and began plotting his attacks in an apartment the two shared.

She did not attend his trial, citing health reasons, but had visited him in prison, according to media reports.

Accusing the Labour party of facilitating multiculturalism, Breivik opened fire on the summer camp of the party's youth wing on July 22nd, 2011, killing 69 people, most of whom were teenagers.

His attack began in Oslo, where he set off a massive bomb outside the main government building, killing eight.