LONDON - Britain apologised on Thursday for contributing to the mistreatment of Abdel Hakim Belhaj, a former Islamist fighter turned politician who was kidnapped and handed over to Libya in 2004 and subsequently tortured, in an unprecedented admission of culpability.

"The UK government's actions contributed to your detention, rendition and suffering," Prime Minister Theresa May said in a letter to Belhaj and his wife Fatima Boudchar, which was read out in parliament by Attorney General Jeremy Wright.

"On behalf of Her Majesty's government, I apologise to you unreservedly," the letter read.

Belhaj, who became Tripoli's military commander after Libyan leader Moamer Kadhafi was ousted in the 2011 revolution, claimed British complicity in the couple's capture by the CIA and later torture by his regime.

The British government accepted the couple "were subjected to a harrowing ordeal which caused them significant distress", and said that it had reached "a full and final settlement" with them both.

Boudchar will receive £500,000 (570,000 euros, $670,000) compensation, but Belhaj did not seek any financial settlement, only an apology.

At a news conference in Istanbul, where he lives, Belhaj expressed "his gratitude for this courageous move".

"This apology is accepted and it puts an end to years of suffering," he said.

In an earlier statement, he argued Britain had "set an example for other nations to follow".

"For more than six years I have made clear that I had a single goal in bringing this case: justice. Now, at last, justice has been done," he said. "A great society does not torture; does not help others to torture; and, when it makes mistakes, it accepts them and apologises."

Boudchar was four-and-a-half months pregnant when she was kidnapped, telling the Guardian that she had been taped to a stretcher for the 17-hour flight to Tripoli.

She was released shortly before giving birth.

"This is a historic day," Boudchar told reporters outside Britain's parliament.

"My message to the world's governments is to support solidarity with no injustice to others."

Belhaj was held for more than six years and said he was subjected to torture.

"Your accounts were moving and what happened to you is deeply troubling," wrote May.

"The UK government believes your account, neither of you should have been treated in this way".

The prime minister added that "we should have understood sooner the unacceptable practices of some of our international partners. We sincerely regret our failures."