Greenpeace and other groups say the rights of the islanders, who cannot return to the archipelago, have been violated

Leading conservation groups have condemned the government's "huge violation" of the rights of thousands of exiled Chagossian islanders who cannot return to their Indian ocean coral islands because they have been surrounded by the world's largest marine nature reserve.

Proposals by the foreign secretary David Miliband Britain in 2008 for the creation of a giant 1m ha marine protection zone closed to all fishing around the almost pristine tropical archipelago were backed enthusiastically by nine of the world's major green groups, including Kew Gardens, the RSPB, Greenpeace, the Pew Environment group, the Zoological Society of London and the Marine Conservation Society. Together they asked supporters to back the Foreign Office proposal for the reserve and raised over 275,000 signatures. The park was finally established in 2010.

But diplomatic cables published by WikiLeaks in 2011 suggested that Britain and the US lured the environment groups with the offer of the reserve and then used its ban on fishing to ensure that no Chagossian would ever be able to live within hundreds of miles of Diego Garcia. This, the largest island in the Chagos archipelago, was cleared by Britain of 1,500 native people in 1964 when it leased the island to the US for a massive military base.

Even if the Chagossians won the legal right to return, they might be unable to live on the islands if they were not allowed to fish.

The apparent hoodwinking of the conservationists seemed to be confirmed by the US diplomatic cable dated May 2009. A British Foreign Office official told the US government that the decision to set up the reserve would "effectively put paid to resettlement claims of the archipelago's former residents".

In further revelations this month, British archives disclosed how the Foreign Office noted in 1966: "The object of the exercise is to get some rocks which will remain ours […] there will be no indigenous population except seagulls".

But some conservation groups who strongly opposed people living on Chagos islands in 2009 now say that they would not object to the islanders returning. "We have no opposition to the return of the Chagossians. We would support this as much as the reserve itself. If there were a return we would support it. I do not think anyone in the coalition ... would oppose a return. I don't think the environment network [of the nine groups] would oppose that in any way," said Marine Conservation Society biodiversity chief, Peter Richardson.

Greenpeace strengthened calls for the human rights of the Chagossians to be respected. "The Chagossian people have suffered, and continue to suffer, a huge violation of their human rights. It is completely within the powers of the UK government to decide to right the historic wrong and agree for the right of the return to the Chagossian people and commit to the development of a joint management plan for the marine reserve than includes the zoning of some areas to enable sustainable subsistence fishing," said Kumi Naidoo, executive director of Greenpeace International, at the Oxford Amnesty Lectures this month.

The TV presenter and conservationist Ben Fogle, one of a few people to have illegally visited the Chagossian islands in the past few years, said he hoped to appeal directly to the US secretary of state, Hilary Clinton. "We fight tooth and nail to save animals from extinction. Do we not owe it to our own people? We have already let the dodo die out, we can't and mustn't let this happen to a people and their culture."

In an article for the Guardian, he says: "I am ashamed to be British. It is a story of deceit and tragedy that has been described by some as the darkest day in British overseas policy. It has shaken my very principles on conservation and democracy. It is a story of deceit that has left thousands of British refugees living in misery for the past 40 years, exiled from their island home by a conniving and unrepentant government."

The European court of human rights is expected to rule by July on whether the islanders have a right to return and a separate judicial review will rule on whether the British government conducted a proper consultation for the reserve, having not shown it to many Chagossians in Mauritius and elsewhere. An international tribunal will also rule, possibly in 2013, on whether Britain had the sovereign right to declare the marine reserve.

"We have always recognised the terrible suffering of the Chagossians as a result of the UK government . I am not saying that they should not have the right to return. When the situation changes, the marine reserve should be reassessed," said Jonathan Hall, overseas territory officer of the RSPB.

Other conservation groups told the Guardian they were "neutral" on the question of whether the Chagossians should return. In a statement, the Linnean society said: "The Chagossians' return and conservation are not necessarily exclusive. [But] it's a very pristine area. I can see people would say 'they were used' [but] looked at from a biodiversity perspective, we don't know much about this amazing place. It may have been convenient for government, but [I do not think] we were being used. We have a golden opportunity to preserve and observe," said Elizabeth Rollinson, secretary of the Linnean Society.

"We support conservation initiatives aiming to protect the rich and important biodiversity found in the Chagos archipelago, including Kew's work in plant conservation in the region and the creation of the Chagos Marine Reserve." said Prof Stephen Hopper, director of the Royal Botanic Gardens, Kew.

"Whatever the outcome of the court case we will support the Chagossians. If they are given the right to return [by the courts] we will do our best to provide them with technical expertise. We would be very happy to work with them," said Heather Koldewey, head of global programmes at the Zoological Society of London.

But she added: "People have an impact. It entirely depends on the scale of the people [their return] and what they do. Chagos represents what our oceans should look like. It's important scientifically. It is an extraordinary place."

David Snoxell, former high commissioner to Mauritius, has appealed to the Foreign Office to negotiate a political solution: "A zone for sustainable fishing by Chagossians should have been designated just as the marine protected area allows the 4,000 personnel on the US base to continue their leisure fishing activities. Two years on it has hardly progressed beyond a paper park and cannot do so until the court cases are settled.

"The Foreign Office should come out of their bunker and negotiate a political compromise. It is a waste of tax payers' money that could have gone into conservation and helping those Chagossians who want to return.

"There is nothing to stop FCO modifying the full no-take area. It would not damage the marine protection area."