At first it was thought David Colly, 48, had died of natural causes at Wayland prison near Watton, Norfolk, on Wednesday night.

But on Friday night officers said a post-mortem examination at Addenbrookes Hospital, in Cambridge, had shown that he had suffered a head injury which led to a fatal brain haemorrhage.

A 29-year-old fellow inmate has been arrested and detained at Thetford police station.