A Libyan rebel commander has claimed UK intelligence agents knew he was being tortured but did nothing to help him.
Abdul Hakim Belhadj said he clearly signalled his physical abuse during an interview in Tripoli but nothing was done to stop the violence against him.
Belhadj is threatening to sue the British Government for its alleged involvement in his 2004 rendition and subsequent imprisonment.
Documents uncovered by Human Rights Watch in the offices of Libyan dictator Muammar Gaddafi's former security chief Musa Kusa suggested that the UK was involved in helping Libya detain terror suspects including Belhadj.
Belhadj told The Times that during a meeting with three British agents he made chopping motions with his hands to signal his treatment.
He said "they moved their heads and agreed... they got my message" and added that "I have no doubt, not a single doubt, they knew", but the torture continued.
Belhadj claimed he was locked in solitary confinement, hung up, beaten and denied a shower for three years.
According to one of the documents found by Human Rights Watch MI6 dispatched an intelligence officer to Tripoli after Belhadj's detention to obtain information of "urgent importance" from him relating to UK anti-terrorist operations.
At the time, he was a member of the Libyan Islamic Fighting Group, which was affiliated to al Qaida.
His spokesman Anes el-Sharif told The Daily Mail: "He may one day take legal action. His suffering is very difficult to forget and he deserves a clear apology from the US and UK."
An independent inquiry is to look into claims that the UK traded information with Gaddafi's security forces in return for intelligence extracted from detainees in the country's jails.
Prime Minister David Cameron told MPs that the allegations were "significant" and would be examined "very carefully" by Sir Peter Gibson's inquiry, which was set up last year to consider allegations of UK complicity in the mistreatment of terror suspects in Guantanamo Bay.
He said that the former Labour administration may have been too "credulous" and "gullible" in its dealings with Gaddafi's regime, but cautioned that there should be no rush to judgment on the security and intelligence agencies MI5 and MI6, who he praised for keeping the UK safe.
A Detainee Inquiry spokesman said: "The Detainee Inquiry is looking at the extent of the UK Government's involvement in, or awareness of, improper treatment of detainees - including rendition.
"We will therefore, of course, be considering these allegations of UK involvement in rendition to Libya as part of our work. We will be seeking more information from Government and its agencies as soon as possible."
Meanwhile Foreign Secretary William Hague said that the UK had sent a diplomatic team to Tripoli including staff from the Foreign Office and Department for International Development.
Mr Hague said the arrival of a diplomatic team in Tripoli "marks another significant step in the UK's relations with the new Libya".
He added: "It will help strengthen relations with the National Transitional Council authorities and support their efforts to rebuild Libya. The team will also liaise with international organisations in order to help address humanitarian needs in Tripoli and the surrounding areas."
Meanwhile, the chair of Parliament's cross-party Intelligence and Security Committee (ISC), Sir Malcolm Rifkind, said he was seeking "clarification" from MI5 and MI6 about "the nature and extent of intelligence-sharing with the Libyan security services and the rendition of Libyan nationals".
Sir Malcolm said the ISC was "considering allegations made about the relationship between the UK security and intelligence agencies and the Libyan security services".
As rebel forces converged on Gaddafi's last strongholds in Libya, Mr Cameron warned the House of Commons that the people of the north African country will not be safe so long as the tyrant remains at large.
In a statement to MPs, he insisted that there must be "no bolt-hole, no pampered hiding place from justice" for Gaddafi, who should "face the consequences of his actions, under international and Libyan law".
Mr Cameron vowed that Britain would not let up on its military efforts "until the job is done", though aides later stressed that no UK troops were being sent in to help in the hunt for Gaddafi on the ground.
Libya's rebel forces have extended to Saturday a deadline for the surrender of Gaddafi's hometown of Sirte and other loyalist areas, and thousands of troops have massed outside the town of Bani Walid, where several prominent members of the former regime are thought to be holed up.
Meanwhile, Colonel Gaddafi's spokesman claimed the dictator was still in Libya and in "high spirits".
Moussa Ibrahim told Syrian-based television station Arrai TV: "The leader Colonel Muammar Gaddafi is in excellent health in high spirits.
"He is present and well and in good health and in a place that can't be reached by those fractious groups and he is in Libya."