A Libyan rebel leader who was rendered to Tripoli with the assistance of MI6 said on Monday that he had told British intelligence officers he was being tortured but they did nothing to help him.

In a claim that will increase the pressure for further disclosure about the UK's role in torture and rendition since 9/11, Abdul Hakim Belhaj said a team of British interrogators used hand signals to indicate they understood what he was telling them.

"I couldn't believe they could let this go on," he said. "What has happened deserves a full inquiry."

Belhaj was detained by the CIA in Thailand in 2004 following an MI6 tipoff, allegedly tortured, then flown to Tripoli, where he says he suffered years of abuse in one of Muammar Gaddafi's prisons.

It emerged on Monday that MI6 had been able to tell the CIA of his whereabouts after his associates informed British diplomats in Malaysia that he wished to claim asylum in the UK. Belhaj was then allowed to board a flight for London and abducted when the plane called at Bangkok.

There were signs that the discovery of a cache of secret MI6 and CIA documents at an abandoned government office building in Tripoli was triggering panic in some parts of Whitehall.

The papers detail the UK's role not only in the rendition of Belhaj, but in that of a second man, known as Abu Munthir. This operation appears to have been planned by British and Libyan intelligence officers without any CIA involvement.

David Cameron said the disclosures would be investigated by the Gibson inquiry, set up last year to examine the UK's role in torture and rendition.

It was unclear whether MI6 or MI5 had disclosed anything to the inquiry before the documents came to light. Inquiry staff first indicated they knew nothing about the Libyan operations, and were seeking information from the government "as soon as possible". Later they said they had "received material relating to these issues", but declined to be more specific.

Similarly, the Conservative MP Richard Ottaway, a former member of the intelligence and security committee, a Westminster body supposed to provide oversight of MI5 and MI6, indicated the committee knew nothing about the UK-Libya operations before giving the agencies a clean bill of health in a 2007 report on rendition; he then said he could say nothing about the matter [see footnote].

The BBC last night reported that Abdelati Obeidi, a former Libyan foreign minister under Muammar Gaddafi, as saying that MI6 had been operating in Tripoli until the start of the revolution in February. He added that relations between Libyan and British intelligence services were not as close as when his predecessor Moussa Koussa was in charge.

Belhaj on Monday revealed more details of the lead-up to his rendition on 6 March 2004, which he says came amid his attempts to reach the UK, of which the government had become aware.

He said he had first tried to travel to London from Kuala Lumpur via Beijing in late February that year. However, he was refused permission to board in Beijing, despite carrying a French passport, which does not require a pre-issued UK visa.

He was returned to Kuala Lumpur where he was detained by Malaysian immigration officials. It is understood that an associate of Belhaj then visited the British embassy in Kuala Lumpur advising officials there of his intention to seek political asylum in the UK.

Shortly afterwards he was freed from the detention centre and allowed to buy a ticket to London via Bangkok. By then he had disposed of his French passport, issued to a Jamal Kaderi, and was travelling on a Moroccan passport, issued in the name of Abdul al-Nabi. Holders of Moroccan passports require a pre-issued visa to enter the UK, but Belhaj said he did not apply for a visa and was allowed to board without one – a highly unusual practice.

The revelation raises fresh questions about the extent of the government's role in Belhaj's rendition. Documents discovered last Friday reveal that a senior MI6 officer, Mark Allen, had written to Koussa, who was then the Libyan spy chief, congratulating him on receiving Belhaj and acknowledging that "the intelligence was British". "I would not board until they assured me that I could travel to the UK," Belhaj said. "They did that and I got on the plane."

Belhaj was captured by CIA officers, in co-operation with Thai authorities, inside Bangkok airport. He says he was tortured at a site in the airport grounds and then sent to Libya, where Gaddafi had long seen him as one of the biggest threats to his tyrannical four-decade rule.

"The British were the second team to visit me," he said. "They came about a month after I was returned to Libya and they were very well briefed about LIFG [Libyan Islamic Fighting Group] members in the UK. They knew everything, even their code names. They wanted to know more details about the LIFG and also about the general environment elsewhere, al-Qaida, that sort of thing. There was a woman who was leading the team, a big man and a third person who was translating. They only came one time."

Belhaj said intelligence officers from other European countries, including France, Germany and Italy, also travelled to Tripoli to speak to him inside the infamous Abu Selim prison in south Tripoli.

Before each visit he was told by Libyan officers – and sometimes by Koussa – to "tell the British and others that the people they are asking about are al-Qaida".

"The Libyans told me that if I told them that I would be treated better."

He said Koussa, who fled the Gaddafi regime in March with MI6 help, would often taunt him in prison, with threats that he would die there. On one occasion Koussa ordered guards to put a shade over half of Belhaj's cell window, to reduce what little sunlight he was getting.

Files seen by the Guardian on Sunday inside the now ransacked offices of the external security service reveal that Libyan spies remained in close co-operation with the CIA and MI6 as late as last November. The files reveal the Americans, in particular, were regularly requesting information about the identities of Libyan mobile phone users. One document showed that the CIA had responded to a Libyan request about the user of a satellite phone by giving GPS references for every call made.

Key questions

Who authorised the UK-Libya rendition operations?

Government ministers gave the go-ahead for MI6 to mount the operations, but the Foreign Office, the Cabinet Office and Downing Street are all declining to say which department's ministers were responsible.

Were there any other rendition operations involving the UK ?

It seems unlikely that there were only two operations, given how extraordinarily close MI6 and Gaddafi's intelligence services appear to have become, but no further details have yet emerged.

Were the British involved in renditions involving other countries?

Papers disclosed in UK courts show that the British government decided to allow its nationals to be "rendered" from Afghanistan to Guantánamo Bay by the US after 9/11, and were involved in the interrogation of prisoners held by agencies known to use torture in Pakistan, Bangladesh and elsewhere. But details of the UK-Libya rendition operation would not have emerged had the Gaddafi regime not collapsed in such a disorderly way, raising concerns the UK government may have been involved in many more operations that remain hidden.

What happened to "Abu Munthir", whose rendition from Hong Kong to Tripoli was planned by the British and the Libyans, according to a secret CIA document discovered at the weekend?

He is now thought not to be the man by that name who was involved in a UK bomb plot in the UK, but another man, Sami Saadi, who was freed from jail in Tripoli last year. There are unconfirmed reports that he is in Tunisia, receiving hospital treatment. But the whereabouts of his family remain unclear.

Will the Gibson Inquiryget to the truth about the UK's role in torture and rendition?

Human rights groups and lawyers representing rendition and torture victims are boycotting the inquiry, which they have condemned as a sham, and now backbench Tory MPs are also saying they are losing faith. Even if Gibson does uncover new information, it's unclear that this will ever be shared with the public – the cabinet secretary, Sir Gus O'Donnell has the final say on what is published and what is kept secret.

• This article was footnoted on 2 November 2011. The main article above named Richard Ottaway, MP, as indicating that the Intelligence and Security Committee was unaware of the cases of two renditions to Libya. Some readers may have inferred that he was the source of the ISC position. He was not and his name was wrongly included in the story due to an editing error.