Clooney takes the biggest risk of his career with film on Bin Laden's driver

He boasts of being Barack Obama's "BlackBerry buddy", never mind a UN Commissioner for Peace and a driving force behind the Save Darfur campaign.

In the latest incarnation of his irrepressible political instincts, the Hollywood actor George Clooney may find himself gnawing at a raw nerve. He has bought the film rights to a book chronicling the life and trial of Salim Hamdan, the Yemen-born driver and bodyguard of Osama bin Laden who was jailed last week for five-and-a-half years for supporting terror.

The Challenge, written by the American journalist Jonathan Mahler, documents the long campaign by the US navy lawyer Charles Swift and the Georgetown law professor Neal Katyal to ensure a fair trial for Hamdan.

Clooney, who has starred in such overtly political films as Syriana and Good Night, and Good Luck, paid an undisclosed sum, probably more than seven figures, on behalf of Smoke House, the production company he founded with the actor and film producer Grant Heslov.

Mr Mahler said last night that he was "really excited", adding: "George Clooney is clearly an outstanding film-maker and I've got no doubt he's going to do a fantastic job with this."

Hamdan's sentence was the first to be delivered in a full war crimes tribunal at Guantanamo Bay, the American prison in Cuba. Proponents of Washington's "war on terror" considered the 66-month jail term too lenient; human rights activists protested that he had served his time – he had already spent seven years in US custody.

American gossip columns buzz with rumours that Clooney – whose performance as the brooding, eponymous hero of legal thriller Michael Clayton earned him an Oscar nomination last year – will play Mr Swift himself. Should he choose to do so at such a politically loaded time, and were he to portray Mr Swift sympathetically, Clooney could be making the most overtly political intervention of his career to date.

"I think Swift has taken a lot of people by surprise," Mahler said. "He had been a minor lawyer defending wayward service personnel on things like child molestation charges.

"Suddenly he's appointed by the Pentagon to defend an enemy combatant, and he ends up suing George W Bush. It's difficult not to be impressed by his sense of duty, by the idea that justice transcends the allegiances of war. I'm sure that's part of the appeal to George Clooney."

Mahler said the story fascinated him because Hamdan , 40, and Mr Swift, "had this extraordinary relationship... Suddenly a man in a military uniform walks into his room one day and says 'Hi, I'm Charlie Swift. I'm your lawyer. You can trust me.'

Hamdan would go on hunger strike after hunger strike, and here was this man in a uniform literally trying to feed him and stop him from going mad. Several times Hamdan tried to sack the man who wanted to save him, but Swift refused to budge."

During Hamdan's sentencing, Mr Swift appealed for his client to be able visit his family in Yemen. Rather than reject the request, Captain Keith Allred, the military judge, said it was a good idea, saying he hoped Hamdan would see his family soon.

The Challenge portrays Mr Swift as a noble David, defeating the ignoble Goliath of Washington's neo-cons. His story has the ingredients of a classic Hollywood yarn: emerging from obscurity, Mr Swift was expected to mount the sort of legal case befitting a man of his inexperience. Instead, he took Hamdan to the Supreme Court – and won. And yet the victory cost him his marriage and led to his being overlooked for promotion.