The Sherlock star has signed on to develop a film about suspected terrorist Mohamedou Ould Slahi, who has been imprisoned at the Guantanamo Bay detention facility in Cuba without ever being charged with a crime or being able to defend himself in court.

Guantanamo Diary, based on Slahi's memoirs, will be produced through Benedict and Adam Ackland's company SunnyMarch. It will detail allegations he made about being subjected to enhanced interrogation techniques and torture.

The movie news comes on the same day Slahi's legal team learned his case would be prioritised by officials at the Periodic Review Board, who will determine if he should be released.

"Mohamedou's co-counsel and I are flying to Guantanamo... with the first good news in almost six years," his attorney Nancy Hollander says. "In 2010, a respected federal judge ordered Mohamedou's release, but the government appealed that order and Mohamedou's case has been in limbo ever since.

"We recently learned that his case will be reviewed by the Periodic Review Board, which provides a possible path to clear him for release, meaning he could be released this year. He will get a chance to speak directly to the panel members, who are drawn from several intelligence agencies. Mohamedou has been detained 15 years without any charges. It is long past time for his release."

Benedict's first production project was 2013 short film Little Favour.