Rapists, paedophiles and other dangerous attackers are expected to discuss their crimes with other inmates as a condition of release.
But Muslim prisoners complain that criminals should not have to talk about their offences - a "legitimate Islamic position", according to Ahtsham Ali, the Prison Service's Muslim adviser.
One thousand inmates were put on the Sex Offender Treatment Programme last year with places usually reserved for the most dangerous attackers.
Failure to complete the course can weigh against an offender at parole hearings.
The possibility of an exemption for Muslims came to light in a letter from an unnamed inmate to Inside Time, a newspaper for prisoners.
The convict said: "I have always insisted that it was against Islamic teachings to discuss your offence to anyone, let alone act it out within a peer group."
Experts said ministers, who have launched a review of the issue, could face a legal challenge from Muslims kept longer in prison because they had not undergone the treatment programme.
Harry Fletcher, of the probation union Napo, said: "If they do not take part, Muslim sex offenders are likely to serve longer sentences, possibly the whole of their term, before they are released."
Nick Herbert, Tory justice spokesman, said: "There can be no religious discrimination when it comes to deciding the appropriate and safe time to release a prisoner.
"In any case, all prisoners, regardless of religion, should have their release made conditional on their behaviour and progress in custody, not be given automatic release at the half-way mark."
A Prison Service spokesman said officials were seeking to ensure the treatment programme was sensitive to "the diversity of religions within the prison context".
"Membership of a particular religion is not a bar to participation in accredited programmes," she insisted, adding that offenders were carefully vetted to ensure they were suitable candidates for the course.