British prisoners are increasingly being forced to convert to Islam by their fellow inmates, the prison officers’ union has warned.

Muslim gangs are growing in power and influence among prisoners, and there are concerns that they are targeting vulnerable new arrivals – making prisons a breeding ground for extremism.

“It is a concern, and there’s been clear evidence from a variety of different incidents. Young men are being targeted and then coerced into converting to Islam,” the association’s general secretary, Steve Gillan, told Sky News.

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One woman, whose brother is being bullied by a gang trying to force him to convert, told the broadcaster: “He just looks like a broken man ... he's tearful on visits. I'm just really scared for him. 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.”

Joe Chapman, a former prison officer who now acts as a prison law consultant, said he thought the situation was deteriorating.

“I think it could be a huge problem. Previously I'd probably only worked in about a dozen or so prisons as an officer, 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.”

The former Home Secretary Lord Reid told Sky News that the sense of protection of being in a gang made them appealing to new arrivals, and described jail as “a very, very fertile ground for recruitment and proselytization and radicalisation”.

A Ministry of Justice spokesperson said: “Faith can have a very positive influence on prisoners and can play an important role in rehabilitation, but it must never be misused as a way of coercing vulnerable prisoners into criminal behaviour.

“We are never complacent about bullying in prisons and take a zero tolerance approach to tackling victimisation of any kind.”