"The British government has probably been assured by the Americans that they're not up to anything there, but those assurances, I'm afraid, are worth nothing"
Clive Stafford-Smith, Reprieve
In a dossier sent to the UK parliament's foreign affairs committee that will invest, Reprieve says boats moored off Diego Garcia may also have been used as floating prisons.
"There have been repeated, credible and concurrent claims that Diego Garcia has played a major role in the US system of renditions and secret detention," documents submitted to the committee said.
But Britain's foreign office said in a statement that the US has given repeated assurances "no detainees, prisoners of war or any other persons" have been held on Diego Garcia, or have transited through the island.
British officials insist they must be notified by Washington if any civilians are held on the base.
Stafford-Smith said: "The British government has probably been assured by the Americans that they're not up to anything there, but those assurances, I'm afraid, are worth nothing and we have evidence that contradicts it."
The entire population of the Chagos archipelago, of which Diego Garcia is a part, 2,000 people according to the islanders, 1,000 according to the British government - was relocated between 1967 and 1973.
The islanders were removed after the UK agreed to lease the island to the US for use as an airbase.
Sheikh Mohammed is suspected of planning the September 11 attacks.
Zubaydah, accused of being a link between Osama bin Laden and many al Qaeda cells;
Hambali, a suspected link between bin Laden and regional terror group Jemaah Islamiyah.
In June, a report by Dick Marty, a Swiss senator investigating the issue for Council of Europe, a human rights watchdog, raised the possibility that the island-territory was used as a processing centre for detainees.
"One of the great tragedies with this whole experiment with abuse over the last five years, is that I think now, my clients, the suspected terrorists, many of whom are innocent, are found more credible that the United States' president," Stafford-Smith said.