One of the more sordid and long-running stories in Anglo-American colonial history – that of Diego Garcia, the chief island of the Chagos Archipelago in the Indian Ocean – reared its ugly head again on Friday when the UK’s all-party foreign affairs committee announced plans to investigate long-standing allegations that the CIA has, since 2002, held and interrogated al-Qaeda suspects at a secret prison on the island.

The shameful tale of Diego Garcia began in 1961, when it was marked out by the US military as a crucial geopolitical base. Ignoring the fact that 2,000 people already lived there, and that the island – a British colony since the fall of Napoleon – had been settled in the late 18th century by French coconut planters, who shipped in African- and Indian-born labourers from Mauritius, establishing what John Pilger called “a gentle Creole nation with thriving villages, a school, a hospital, a church, a prison, a railway, docks, a copra plantation,” the Labour government of Harold Wilson conspired with the administrations of Lyndon Johnson and Richard Nixon to “sweep” and “sanitize” the islands (the words come from American documents that were later declassified).

Although many islanders traced their ancestry back five generations, a British Foreign Office official wrote in 1966 that the government’s aim was “to convert all the existing residents ... into short-term, temporary residents,” so that they could be exiled to Mauritius. Having removed the “Tarzans or Men Fridays,” as another British memo described the inhabitants, the British effectively ceded control of the islands to the Americans, who established a base on Diego Garcia, which, over the years, has become known as “Camp Justice,” complete with “over 2,000 troops, anchorage for 30 warships, a nuclear dump, a satellite spy station, shopping malls, bars and a golf course.” So thoroughly were the islands cleared – and so stealthy the procedure – that in the 1970s the British Ministry of Defence had the effrontery to insist, “There is nothing in our files about a population and an evacuation.”

Suffering in exile, the Chagos islanders have struggled in vain to secure the right to return to their ancestral home, winning a stunning victory in the High Court in 2000, which ruled their expulsion illegal, but then suffering a setback in 2003, when, with typically high-handed authoritarianism, Tony Blair invoked an ancient and archaic “royal prerogative” to strike down their claims once more. Although the appeal court reversed this decision in May 2006, ruling that the islanders’ right to return was “one of the most fundamental liberties known to human beings,” it remains to be seen how this belated judicial recognition of their rights can be squared with the Americans’ insistence that their military-industrial archipelago must remain unsullied by outsiders.

In their resistance to the islanders’ claims, Blair and the Foreign Office were clearly protecting the interests of their American allies, for whom the geopolitical importance of Diego Garcia as a strategic base had recently been augmented by its use – and the use of some of the ships moored there – as fabulously remote offshore prisons in which to hold and interrogate “high-value” al-Qaeda suspects.

The suspicion, which the foreign affairs committee has pledged to investigate, is that on Diego Garcia the Americans found a far more compliant partner in torture – the British government – than they found in most other locations chosen for secret CIA prisons. According to various reports over the years, the Americans’ other partners in the offshore torture game – Thailand, Poland and Rumania, for example – were only prepared to be paid off for a while before they got cold feet and sent the CIA packing.

Whether the committee will probe deeply or not remains to be seen. The British-based legal charity Reprieve, which has called for such an investigation for some time, has already told the committee in a submission that it believes that the British government is “potentially systematically complicit in the most serious crimes against humanity of disappearance, torture and prolonged incommunicado detention.” Clive Stafford Smith, Reprieve’s legal director, told the Guardian that he is “absolutely and categorically certain” that prisoners have been held on the island.

When questioned by diligent MPs like Andrew Tyrie, the Conservative MP for Chichester, who is a staunch opponent of the CIA’s use of “extraordinary rendition,” the British government has persistently maintained that it believes “assurances” given by the US government that no terror suspects have been held on the island, but there are several compelling reasons for concluding, instead, that the government is actually being economical with the truth.

Studies of planes used by the CIA for its rendition program have established that on September 11, 2002, the day that 9/11 plotter Ramzi bin al-Shibh was seized after a firefight in Karachi, one of the CIA’s planes flew from Washington to Diego Garcia, via Athens. Bin al-Shibh did not resurface again until September 2006, when he was moved to Guantánamo, and he has not spoken about his experiences. Unlike his supposed mentor Khalid Sheikh Mohammed, he refused to take part in his tribunal at Guantánamo earlier this year, but this is not the only piece of the torture jigsaw that has been reconstructed by diligent researchers.

In June 2006, Dick Marty, a Swiss senator who produced a detailed report on “extraordinary rendition” for the Council of Europe, also concluded that Diego Garcia had been used as a secret prison. Having spoken to senior CIA officers during his research, he told the European Parliament, “We have received concurring confirmations that United States agencies have used Diego Garcia, which is the international legal responsibility of the UK, in the 'processing' of high-value detainees.”

Anecdotally, Marty’s findings have been confirmed by other sources. Manfred Novak, the UN’s Special Rapporteur on Torture, declared that he heard from “reliable sources” that the US has “held prisoners on ships in the Indian Ocean,” and detainees in Guantánamo have also told their lawyers that they were held on US ships – in addition to those held on the USS Bataan and the USS Peleliu, which are discussed in my newly-published book "The Guantánamo Files." One detainee told a researcher from Reprieve, “One of my fellow prisoners in Guantánamo was at sea on an American ship with about 50 others before coming to Guantánamo. He told me that there were about 50 other people on the ship; they were all closed off in the bottom. The people detained on the ship were beaten even more severely than in Guantánamo.”

The most incriminating evidence of all, however, has come not from opponents of Guantánamo, or, indirectly, from those subjected to some of the regime’s most horrendous abuses, but from an upstanding insider. Barry McCaffrey, a retired four-star US general, who is now professor of international security studies at the West Point military academy, has twice let slip that Diego Garcia has, as the administration’s opponents have struggled to maintain, been used to hold terror suspects. In May 2004, he blithely declared, “We're probably holding around 3,000 people, you know, Bagram air field, Diego Garcia, Guantánamo, 16 camps throughout Iraq,” and in December 2006 he slipped the leash again, saying, “They're behind bars ... we've got them on Diego Garcia, in Bagram air field, in Guantánamo.”

Do we need any further proof?