Prisoners in UK jails are being bullied and intimidated into converting to Islam, say experts.

The number of Muslims in British jails rose recently to 11,200, a 200% increase on 3,681 in 1997, and prisoners have spoken of a reign of terror in some jails.

The sister of one inmate serving 10 years in a high security prison told Sky News: "He just looks like a broken man ... he's tearful on visits. I'm just really scared for him."

She claimed he had been subjected to escalating violence: "He's been physically assaulted. He's had black eyes.

"In the showers, he got threatened with a knife. He's not going to back down. He's not going to convert for anyone."

She added: "He just spends his time in hiding in his cell. He's got at least another five years to serve. I don't know how much longer he can hold out."

It is believed that about 13% of the prison population are now Muslim, compared with 5% of the population as a whole. A majority are either inmates with Asian, African, or Middle Eastern backgrounds, but a growing number are converts.

Prison officials have spoken of a gang culture inside some UK jails, with Muslims aggressively forcing inmates to convert to swell their ranks and strengthen their hold on crime rackets.

Many also fear that radicalised inmates could commit terrorist acts in the future: prison guards at one Cambridgeshire institution said that inmates who were members of al-Qaida were regarded with "awe" by younger prisoners.

"I think it could be a huge problem. Previously I'd probably only worked in about a dozen or so prisons as an officer," Joe Chapman, a former prison guard turned legal adviser told Sky.

"But this job takes me to 40 or 50 over the year, throughout the country. It's become obvious to me that it's a growing problem.

"About half a dozen of my clients have directly reported problems with being forced to convert ... those that weren't Muslim when they came in and those that were and have been forced to look at more radical ideas about their faith."

Some prisoners have spoken of 'Sharia Law' being enforced in some prisons, with inmates banned from eating pork by Muslims, pictures of women torn down and music systems destroyed.

One letter to the prison journal Inside Time spoke of a regime imposed in Long Lartin prison in Worcestershire.

"The kitchen is usually occupied by 90% Muslims and we have been told if we cook pork we will be stabbed. There have been incidents here where people have been targeted and pressured and bullied into converting to Islam," he wrote.

"Young Muslim men are being radicalised in here and one day they may commit acts of terrorism in this country.

"There seems to be nothing being done here to stop it and people are scared to speak out."

A recent Ministry of Justice report found bullying and forced conversions also prevalent in Whitemoor Prison, Cambridgeshire, where about one-third of inmates are Muslims.

The report concludes: "More prominent, in practice, were pressures (and temptations) felt by some prisoners to convert to Islam. Conditions in the prison made participation in Islamic practices the most 'available' option for those looking for belonging, meaning, 'brotherhood', trust and friendship."

Prison Officers' Association national chairman Colin Moses, in an interview with the BBC, warned of the dangers posed by radicalised inmates.

"People are being radicalised, forcibly radicalised by these gangs. We see it as a real danger, now and for the future of prisons.

"As the Muslim population grows, the gangs are becoming more and more prevalent by the week and they fight to take control of the drug trade and the dealing of mobile phones in prison. This will make our prisons even more violent," he said.