Preserved: Britain's 'barrier reef'

Government scheme to protect ecology of Chagos Islands delights green movement, but exiled islanders will be kept away so that US can retain use of its airbase

Britain is to establish the biggest marine nature reserve in the world, centred on the Chagos Islands in the Indian Ocean, the Foreign Secretary, David Miliband, announced last night.

The reserve, which will contain 55 tiny coral islands sitting in a quarter of a million square miles of the cleanest seas on the planet, and supporting a pristine ecosystem teeming with wildlife, will rival the Galapagos Islands and the Great Barrier Reef in ecological richness. It is likely that fishing and other forms of ocean exploitation will be banned in the area.

The announcement was warmly welcomed by a coalition of Britain's leading environmental groups, ranging from the Royal Botanic Gardens at Kew and the Zoological Society of London to the Royal Society for the Protection of Birds and Greenpeace, which has campaigned to preserve the islands in the face of growing threats to the marine environment around the world, especially from pollution and over-fishing.

"We are thrilled by the UK government's decision to declare the Chagos in its entirety as a no-take protected area," said Alistair Gammell, a founding member of the Chagos Environment Network (CEN). "The oceans desperately need better protection, and the UK has secured a conservation legacy which is unrivalled in scale and significance."

However, the announcement brought anger and criticism from supporters of the Chagossians – the islands' original inhabitants, who were forcibly removed en masse between 1967 and 1973, so that the largest inhabited island, Diego Garcia, could be used by the US as an airbase for strategic bombers. Roch Evenor, an islander who chairs the UK Chagos Support Association, described it as "incomprehensible". "They will say that if you go there, you are not allowed to fish," he said. "How are you going to feed yourself? How are you going to get your livelihood?"

The Chagossians are caught up in a long-running legal battle with the British government over their right to return, which will come to a head in a case to be heard before the European Court of Human Rights in Strasbourg in the summer. The Foreign Office announcement makes no mention of them, other than to say that it is made "without prejudice to the outcome of the current, pending proceedings". "This is really quite shocking," said David Snoxell, a former British high commissioner in Mauritius, and deputy commissioner of the British Indian Ocean Territory (BIOT) – the islands' official name – who is now the co-ordinator of the Chagos Islands All-Party Parliamentary Group.

"I deplore the fact that the Chagossians have not been brought into this. It's as if they've been airbrushed out of the press release, and out of existence.

"The Government is clearly not going to take their interests into account, not until the decision at Strasbourg. They are really pre-empting the Strasbourg decision and attempting to influence the court."

The clash of opinions demonstrates how difficult it is becoming to weigh up the protection of the best remaining parts of nature, in a rapidly degrading world, against the needs and rights of people. There is no doubt that environmentally the case for protecting the Chagos Islands is overwhelming: its seawater is the least-polluted ever tested; its coral reefs are entirely unspoilt; its whole ecosystem, with its countless seabirds, turtles, coconut-cracking crabs (the world's largest), dolphins, sharks and nearly 1,000 other species of fish, is pristine.

But the case of the former islanders, originally numbering about 1,500, is also pressing. Now numbering more than 4,000, and living largely around Crawley in Sussex, the Chagossians were given permission to return in 2000 by Robin Cook, when he was Foreign Secretary, after details of their deportation 30 years earlier had come to light.

However, after 9/11, Diego Garcia assumed a new importance for the US: it is used as a base for bombing missions over Afghanistan (and has also been used for the CIA's "extraordinary rendition" flights taking captives around the world for interrogation). As a result, in 2004 the British government reversed Robin Cook's decision, using the Royal Prerogative and bypassing Parliament. A series of court decisions since then has resulted in victory for the Government, and the appeal to Strasbourg is the islanders' last hope.

Compelling as the environmental case is for declaring a no-take marine reserve, it clearly also suits the Government's policy of wanting to keep the islanders from returning. The issue is seen as a difficult one, and it did not go unnoticed yesterday that the announcement was released at the end of the working day on Maundy Thursday, with Parliament closed and many Britons heading off for an Easter break.

Mr Snoxell accused the Government of duplicity, claiming that the Foreign Office minister Ivan Lewis had said in a House of Commons debate on the islands, on 10 March, that he "put on record a commitment to make sure that interested Honourable Members are briefed before we make final decisions on the marine protected area". This had not been done, Mr Snoxell said, adding: "I deplore the side-lining of Parliament during a recess."

Mr Miliband said in his announcement that the marine protected area "will cover some quarter of a million square miles and its establishment will double the global coverage of the world's oceans under protection".

"Its creation is a major step forward for protecting the oceans, not just around BIOT itself, but also throughout the world," he said. "This measure is a further demonstration of how the UK takes its international environmental responsibilities seriously."