Michael Adebolajo, serving a life-sentence for the 2013 murder of a British soldier, Lee Rigby, was rumoured to have been moved to another prison in order to stop him from radicalising other prisoners.
But The (British) Sun newspaper has confirmed that he is instead being kept in a unit which other prisoners say is like living in a submarine.
Sources said the option to move him to Durham’s HMP Frankland from London’s Belmarsh was ruled out.
They said: “There is no way Adebolajo was destined for the same jail as the likes of Anjem Choudary.
Together they would be an explosive mix.” He was taken instead to Wakefield maximum security jail in West Yorks.
The source added: “The move was designed to prevent his malign influence spreading any further.”
The prison has one of the UK’s five “close supervision centres”.
Prisoners exercise alone in tiny cages, see little natural light and spend up to 22 hours a day in cells.