Muslim extremists have turned part of a jail into a “no-go zone,” prison union leaders claim.
They believe an entire block at Gartree is being run under Sharia law.
Non-Muslim inmates at the jail are said to be refusing to move to C unit over fears of Muslim gangs forcing them to convert.
A source said: “It is so bad non-Muslim prisoners are refusing to move there as they feel intimidated.
“There is huge pressure put on them to convert and a threat of violence if they don’t.
“The Muslim gangs have their own rules and use Sharia law to sort out disagreements.
“In effect, they are setting up their own prison within a prison.
“There have been stories about non-Muslim inmates being barred from putting up topless pictures of women in their cells and even showering naked.”
The fears have been raised following a tour of the Leicestershire jail by leaders of the Prison Officers’ Association.
The union has made its views known to the National Offender Management Service, which oversees the prison service.
Islamic terrorists such as shoe bomber Richard Reid and Lee Rigby killer Michael Adebowale are among scores of convicts believed to have been radicalised in jail.
A Prison Service spokesperson said: “There is no evidence to back-up any of these claims about HMP Gartree.
“But we are not complacent about the possible risks that Islamist extremism poses to our prisons. That is why the Secretary of State commissioned an urgent review of how the prison system deals with this issue.”