A lawyer for a prisoner at Guantanamo Bay has slammed reports that Fifty Shades of Grey is a favourite read at the US detention camp in Cuba.

James Connell claimed that the guards at the jail gave his client Ammar al-Baluchi, who is accused of being involved in 9/11, a copy of the book as a joke to discredit him.

Claims about the erotic novel's popularity within the camp were made by US congressman Jim Moran who toured the high-security unit where more than a dozen "high-value" prisoners, including five men charged with plotting the September 11 attacks, were held.

"Rather than the Koran, the book that is requested most by the [Camp 7 detainees] is Fifty Shades of Grey, They've read the entire series," Moran told the Huffington Post.

"It demystifies them. It exposes them for who they actually are," he said. The inmates were "not exactly holy warriors", he added.

Baluchi, 35, who is also known as Abd al-Aziz Ali, turned up in court at Guantanmo for his pre-trial hearings clutching a copy of the EL James's bestseller saying it was a gift from the guards, according to his lawyer.

Connell said his client had not read the book and preferred the Economist and Wired magazines.

"He says, 'No thank you.' He does not want the book," Connell said. "It's in my safe and as soon as I am able I will return it to Joint Task Force Guantanamo."

"He [Baluchi] knew that it was some sort of a joke," said Connell. "Or some sort of disinformation campaign.

"If this is a practical joke it has gone too far."

Baluchi is a nephew of Khalid Sheikh Mohammed, the Guntanamo prisoner who has been described as the mastermind of the 9/11 plot.

Moran's remarks raised suspicion among long-time experts and reporters on Guantanamo such as Carol Resenberg, who described the claims as odd.

Journalists are not allowed to visit Camp 7, a secretive facility that opened in 2006.