A Libyan Islamist and militia leader has started legal action against Britain for what he says was his illegal rendition by British security forces to face torture in Muammar Gaddafi's Libya.

Abdel Hakim Belhadj is a former leader of the Libyan Islamic Fighting Group, which waged an insurgency against Gaddafi in the 1990s. He also spent time with Islamist militants in Afghanistan, but says he was not allied with Osama bin Laden's al Qaeda.

He was taken to Libya in 2004, and says it was British and U.S. intelligence services that delivered him there in secret, against his will.

In Libya, Mr Belhadj was detained for six years in some of the country's most brutal jails and was interrogated by 'foreign' agents, including some from the UK, Reprieve, a British legal charity advising Belhadj, said on Monday.

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. The beatings and inhumane treatment continued until 2010 when he was eventually released.

Documents found in Tripoli after Gaddafi was toppled implicated Britain in illegally sending Belhadj and other terrorism suspects back to Libya, and prompted British Prime Minister David Cameron to launch an independent investigation.

NO APOLOGY

The legal team filed this lawsuit after we asked for a formal apology for handing me and my wife to the former regime, which tortured me mentally and physically, Belhadj told Reuters in Libya.

Because of this, we requested a formal apology from the British government and for my name to be removed from the terrorism list. When they refused, we turned to the courts ...

(Britain's) MI6 were not the only ones involved in this. The CIA also took part.

In September, shortly after the fall of Tripoli to anti-Gaddafi forces, Human Rights Watch said it had found documents in the office of Gaddafi's former intelligence chief Moussa Koussa indicating that U.S. and British spy agencies had helped Gaddafi to persecute Libyan dissidents.

One file implicated the U.S. Central Intelligence Agency in Belhadj's capture and return to Libya, the rights group said.

Reprieve cited another document that it said had come to light after Gaddafi's fall.

In it, Mark Allen, former counter-terrorism chief at the British foreign intelligence agency MI6, tells Koussa:

Most importantly, I congratulate you on the safe arrival of Abu Abd Allah 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.

LIVING IN CHINA

Reprieve said Belhadj had been living in China in 2004 with his pregnant wife before deciding to seek asylum in Britain.

When they tried to leave China, they were detained and deported to Malaysia. Once there, they were told they would be allowed to travel to Britain, but only through Bangkok, Reprieve said.

On reaching Bangkok, the couple were separated, handed over to U.S. authorities and taken to what they believe was a U.S. secret prison. There they were subjected to a barrage of barbaric treatment, Reprieve said.

It said the couple were then secretly flown to Libya by U.S. authorities, and that their plane stopped to refuel on Diego Garcia, a British island territory in the Indian Ocean.

Reprieve said Belhadj was also considering taking legal action against the United States, but was focusing on Britain because of its alleged central role in his transfer.

Belhadj has said he bears no grudges against the British or American people.

I don't find myself seeking revenge, it is not a trait of mine. If there is any reaction, it is going to be according to the law and will be followed up by my lawyers, he told Reuters in an interview last month.

British, French, U.S. and other allied forces were instrumental in ending Gaddafi's rule by enforcing a no-fly zone over Libya to protect civilians from his forces as an uprising raged on the ground.

Britain's relations with Libya thawed during Tony Blair's period as prime minister. He visited Libya in 2004 and announced that Gaddafi was ready to help Britain's fight against terrorism.

The British government confirmed it had received a letter preceding legal action. The Foreign Office said it did not comment on intelligence matters, but that Britain was against torture and did not ask others to conduct torture on its behalf.

(Additional reporting by Matt Falloon)