Norwegian mass killer Anders Behring Breivik played violent video games for up to 17 hours a day in preparation for his brutal massacre of 77 people.

He spent 12 months in isolation from society playing first-person shoot-me-up Call of Duty - where he honed his shooting technique by executing fictional enemies online.

He also took a year off from work to play World of Warcraft - a multi-player role-playing game which has more than 10million subscribers - he told an Oslo court.