Abdel Hakim Belhadj says he was detained for six years, during which he received vicious beatings and a death sentence

A Libyan military commander and rebel leader has launched legal proceedings against the British government over his rendition and alleged "barbaric" treatment meted out to him and his pregnant wife.

Abdel Hakim Belhadj, head of the Tripoli Military Council and a former leader of the militant Libyan Islamic Fighting Group, is suing ministers and MI6 for the part he claims they played in secretly sending him and his wife to Libya in March 2004.

Belhadj was then living in China with his wife, Fatima Bouchar.

They decided to seek asylum in Britain but were detained and deported to Malaysia, where they were immediately taken into the custody. They were told they would be allowed to travel to the UK via Bangkok. They were then handed over to the US authorities and taken to what they say was a US secret prison.

In between interrogation sessions, Belhadj says, he was hung by his wrists from hooks in his cell for prolonged periods while hooded and blindfolded, and viciously beaten.

The couple were then rendered by the US to Libya. Belhadj says he was hooded and shackled to the floor of the plane in a stress position, unable to sit or lie during the entire 17-hour flight, which is believed to have stopped to refuel in Diego Garcia, the British Indian Ocean territory. Neither he nor his wife was aware that the other was on the plane.

In Libya, Belhadj was detained for six years in some of the country's most brutal jails and was interrogated by "foreign" agents. He says he was savagely beaten, hung from walls, and cut off from human contact and daylight before being sentenced to death during a 15-minute trial about four years into his detention.

Belhadj – also known as Abu Abdullah al-Sadiq, a commander of anti-Gaddafi forces in the 2011 Libyan conflict – has instructed the law firm Leigh Day to sue the British government, with the help of the legal charity Reprieve.

Belhadj is the second Libyan dissident to launch legal action against the British government in recent months. In October, Sami al-Saadi launched an action to claim damages from the UK after a cache of secret documents discovered in Tripoli exposed the pivotal role played by MI6 in his rendition to one of Muammar Gaddafi's jails.

Cori Crider, Reprieve's legal director, said: "Mr Belhadj was totally willing to come to an agreement with the British government. He made it absolutely plain that what he cared about was an open apology, and for those who tortured him and his wife to be brought to justice."

Sapna Malik, from Leigh Day, said: "The barbaric treatment which our clients describe, both at the hands of the Americans and the Libyans, is beyond comprehension, and yet it appears that the UK was responsible for setting off this torturous chain of events."

Britain's role in the Belhadj renditions was revealed in a letter from Sir Mark Allen, former director of counter-terrorism at MI6, to Moussa Koussa, the head of the Libyan intelligence agency at the time.

In a letter dated 18 March 2004, Allen said: "I congratulate you on the safe arrival of Abu Abdullah al-Sadiq. This was the least we could do for you and for Libya to demonstrate the remarkable relationship we have built over the years. I am so glad. I was grateful to you for helping the officer we sent out last week."

The Foreign Office says the inquiry set up under a former appeal court judge, Sir Peter Gibson, will consider whether Britain was involved in, or aware of, the improper treatment and rendition of detainees.

Whitehall officials say MI6 told ministers about the rendition of Belhadj and his wife, and will say so in evidence to the Gibson inquiry. However, most of the evidence to Gibson will be held in secret.