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The Prison Officers Association says it is symptomatic of the growing power and influence of Muslim gangs in prison.
However, there are also concerns that some of those converts could be radicalised by more extremist elements in prison.
Sky News spoke to one young woman who said her brother was being bullied by members of a Muslim gang, who were trying to force him into converting to Islam.
The woman did not want to be identified for fear of reprisals against her brother, who is serving a sentence in excess of 10 years in a high security prison in England.
She said: "He just looks like a broken man ... he's tearful on visits. I'm just really scared for him."
She also claimed the bullying had taken a more violent turn: "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."
Official sources acknowledge forced conversions are a problem in the country's prisons.
How large the problem is remains unclear, as inmates are often afraid to report such intimidation for fear of reprisals. more here ... http://goo.gl/F946EW