GETTY Jizya is thought to be inspired by ISIS who tax non-Muslims in Iraq and Syria

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The inmates inside the category A prisons are having to pay the "tax" – known as "jizya" – because they fear being attacked by the extremists. Jizya was found to be enforced by some Islamist extremist prisoners inside four of Britain's largest prisons. The terrifying findings were revealed by a team of Government investigators, appointed by justice secretary Michael Gove to establish the threat Islamist extremists pose in Britain's prisons.

GETTY Inmates inside Belmarsh prison were found to be paying the tax

The investigation – which has been running since last August – suggests that prisoners are being bullied and violently threatened into paying the money or converting to Islam. Inmates often use tobacco and other items smuggled inside prisons to pay the extremists. But some victims said they have been forced to ask family and friends for money to help pay the tax.

One Whitehall source said the tax could be inspired by the actions of ISIS. The wealthy terror group is known to demand jizya from non-Muslims living in Iraq and Syria. The source said: "It sets a dangerous precedent and sends a message to non-Muslim inmates that Muslims are going to run the prison according to their own rules and sharia."

GETTY Extremist Abu Qatada was held at Belmarsh prison before being deported to Jordan

Some inmates have even been pressured into converting to Islam to ease their time at the prisons, rather than paying the tax. The investigation team examined Belmarsh, Long Lartin, Woodhill and Whitemoor prisons – all of which are Class A prisons that hold a total of 2,633 prisoners. Belmarsh prison has previously held numerous high profile Muslim clerics, including Abu Qatada who was held there before being deported to Jordan.

AP ISIS is believed to have amassed millions in funding and taxes

Will it go away? No. I think the assumption is that it will get worse Steve Gillian

General secretary of the Prison Officers' Association Steve Gillian said prisons across the UK are currently struggling to cope with the radicalisation of inmates. He said: "People at times are bullied and intimidated and harassed into becoming a Muslim and, if they don't, are attacked. "There is a massive issue about radicalisation and extremism and, to be fair, the prison service is trying to address it.. Will it go away? No. I think the assumption is that it will get worse.