A former prisoner who claims that he took part in Sharia courts and punishment beatings has given a detailed account of how he came to join a group of prisoners at HMP Woodhill, Milton Keynes, who pledged allegiance to Isis.

The revelations have prompted security experts to call for a fundamental review of terrorist radicalisation in jails across Britain.

The issue came under scrutiny last month when Usman Khan, 28, a Category A terrorist prisoner freed on licence, murdered two people at London Bridge before being killed by police.