Chris Grayling was warned that he could face legal proceedings over the prisoner book ban today, as the campaign against the policy escalated.
Geoffrey Robertson, prominent human rights lawyer and joint head of Doughty Street Chambers, said the Ministry of Justice could be taken to court for acting unlawfully and irrationally and for denying prisoners their right to receive information.
The ban on prisoners receiving books may also breach the 1688 bill of rights by having the secretary of state, rather than a judge, inflict "cruel and unusual punishment" on prisoners who are literate.
He also warned that the exclusion of the Bible from the limit of prisoners having 12 books in their cell could constitute religious discrimination.
"Mr Grayling is not a lawyer; he is a politician who seems to think he is above the law," Robertson said.
"He has no power to impose additional punishment on prisoners over and above that which is imposed by the courts. The action has nothing to do with prison security or any other legitimate purpose.
"The right to read is precious in this country and for prisoners it is a way to lift themselves out of the slough of criminality.
"To deny them the books they need in order to improve themselves is both unreasonable and counter-productive."
Frances Crook, chief executive of the Howard League for Penal Reform, added: "The Ministry of Justice should recognise this policy is flawed and the ban on loved ones sending in books and other essentials to prisoners has little or no support from anyone with an ounce of common sense.
"Reading books goes hand in hand with education, with rehabilitation, with humanity. The spurious arguments in favour of this measure simply don't stand up to scrutiny."
The Ministry of Justice says the ban is part of its earned privileges scheme.