Muslim extremists at a Leicestershire jail have turned part of the prison into a “no-go zone” for non-Muslims, and are running an entire block under Sharia law, prison union leaders have claimed.

Non-Muslim inmates at HMP Gartree, a maximum security prison, are said to have to conform to rules set by Muslim inmates, and are refusing to be moved to the block in question for fear of being forced to convert to Islam. The fears were raised following an inspection of the prison by leaders of the Prison Officers’ Association.

A source told The Sun: “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.”

HM Gartree houses a number of notorious Islamic terrorists, including Michael Adebolawe, who hacked to death Fusilier Lee Rigby outside his barracks in London in 2013, and airplane shoebomber Richard Reid.

But Gartree may not be an isolated case — the revelation comes just days after Lord Falconer, the shadow justice secretary warned that British prisons are becoming “terrorist academies,” with Islamic gangs in charge of a number of prison wings.

He said that as many as 1,000 Muslim inmates were currently at risk of being radicalised inside British jails, a figure higher than the total number of Brits thought to have travelled to Syria to train as jihadis.

“Everything about the prisons is going wrong at the moment — assaults by prisoners on prison officers; assaults by prisoners on prisoners; prisoner suicides; incidents where riot squads are required to deal with trouble,” Lord Falconer told the Sunday Times.

“If all of those things are going wrong, it is extraordinarily unlikely that [the combating of] radicalisation is also not going wrong,” he added.

“We do not want a situation where vulnerable Muslims are being sent to terrorist academies in our high-security prisons without there being a strategy on how to deal with it.”

Islamic proselytising inside prisons is not a new phenomenon. In 2014 Islamic convert Jordan Horner, who has taken the Islamic name Jamaal Uddin, told the BBC how he converted others to Islam while serving a term for trying to bring Sharia to the streets of East London.

“The prison officers witnessed people become Muslim and in front of them I was giving them what we call Shahada, an invitation and acceptance of Islam,” he said.

“They was becoming Muslim in front of the prison officers – and they [the officers] felt sort of powerless. They said I was trying to divide Muslims from non-Muslims, trying to get them to follow an extreme version of Islam.”

But the wholesale take-over of prison wings has not been spoken of until now. The prison service said:

“We are not complacent in the slightest about the possible risks that Islamist extremism poses to our prisons.”