The government is under mounting pressure to "come clean" about the role of an overseas UK territory leased to the US and allegedly used as a secret "black site" detention centre.

An opponent of Colonel Gaddafi who was rendered in a joint MI6-CIA operation, and a leading human rights group representing him, have demanded that the foreign secretary, William Hague, clarify the UK's position on Diego Garcia, an atoll in the Indian Ocean leased to the US until 2016. The Senate's intelligence security committee is preparing to declassify a file that reportedly confirms that the CIA detained "high-value suspects on Diego Garcia" and that "the black site arrangement on the atoll was made with the 'full cooperation' of the British government".

The revelations are hugely troubling for the government and threaten to raise awkward questions about the UK's relationship with the US, its closest security ally. They strengthen claims made by Abdel-Hakim Belhaj, a rebel military commander and opponent of Gaddafi, who was arrested in Malaysia and rendered with his pregnant wife to Libya, allegedly via Diego Garcia, in a joint US-UK intelligence operation.

Papers discovered in Tripoli in 2011 show that the British security services were instrumental in helping Libya to seize Belhaj, who says he was tortured during his rendition and during his subsequent four-and-a-half-year incarceration by the Gaddafi regime. A flight plan confirmed the CIA had intended to render him via Diego Garcia.

Belhaj, who unsuccessfully tried to bring a case against former foreign secretary Jack Straw, former senior MI6 official Sir Mark Allen, the security services and the Foreign Office, told the Observer that the Senate report raised new questions about the role played by the British overseas territory in facilitating the CIA's extraordinary rendition programme.

"The first time I heard that I had gone through a place called Diego Garcia was when I was told by the head of the Libyan intelligence, Moussa Koussa, during my first interrogation session in a prison outside Tripoli," Belhaj said.

"He was running the interrogation and was angry that it had taken a long time for me to arrive in Libya. I told him that the plane had stopped somewhere on the way from Bangkok. He told me that he knew, and that the plane had landed on an island in the Indian Ocean called Diego Garcia.

"Perhaps he was showing off, or perhaps he had been given wrong information, I don't know. I just know that the flight stopped somewhere. I was chained up in a very painful position and had no means to know where I was, or even whether my pregnant wife – who had been kidnapped at the same time – was with me."

Although the British government admitted in 2008 that two rendition flights carrying detainees had stopped for refuelling on Diego Garcia in 2002, it has consistently denied that detainees were held on it.

"Each year the US government reaffirms to us during our official political-military discussions that all previous assurances since 2008 on this subject remain correct," Mark Simmonds, the minister for overseas territories, wrote in a letter last month to Richard Ottaway, the chairman of parliament's foreign affairs select committee. "Namely that, apart from two instances in Diego Garcia during 2002, there have been no other instances in which US intelligence flights landed in the United Kingdom, UK overseas territories or crown dependencies, with a detainee on board since 11 September 2001."

Polly Rossdale, deputy director at human rights group Reprieve, which has acted for Belhaj, said: "The government must come clean about the UK's role in this dirty affair."

A spokesman for the Foreign Office declined to add any comment to what ministers had already told parliament.