The Chagos archipelago is a group of 55 tropical islands over half a million square kilometres in the middle of the Indian Ocean, about 300 miles south of the Maldives. They have belonged to Britain since they were captured from France in 1814 during the Napoleonic WarsPhotograph: Chagos Conservation Trust

The archipelago boasts the world's largest coral atoll and the world's cleanest, most pristine waters, that are home to at least 220 coral species and more than 1,000 species of fishPhotograph: Dan Jones/Zoological Society of London

The underwater landscape of 6,000m deep trenches, oceanic ridges and sea mounts, is also a refuge and breeding ground for large and important populations of sharks, dolphins, marine turtles, rare crabs, birds and other vulnerable speciesPhotograph: Anne and Charles Sheppard/Chagos Conservation Trust

With 554,000 sq km of reef, the territory would become the largest protected marine area on EarthPhotograph: Anne and Charles Sheppard/Chagos Conservation Trust

Sea fans are abundant on Chagos reefs. Nine conservation and scientific organisations including the Marine Conservation Society, the RSPB, the Zoological Society of London (ZSL) and the Royal Botanic Gardens Kew have formed the Chagos Environment Network (CEN), which is campaigning to protect the biodiversity of the Chagos islands and surrounding waters alongside a three-month public consultation (pdf) launched by the Foreign and Commonwealth Office (FCO) in November 2009Photograph: Anne and Charles Sheppard/Chagos Conservation Trust

Pollutant levels in Chagos waters are exceptionally low because of minimal human influence. Since the 1960s the islands have been set aside for defence purposes, with no inhabitants except for the military personnel and civilian contractors at the US military base on Diego GarciaPhotograph: Dan Jones/Zoological Society of London

A nurse shark. The islands are a refuge and breeding ground for large and important populations of sharksPhotograph: Anne and Charles Sheppard/Chagos Conservation Trust

Because of minimal human impact, the ecosystems of the Chagos have so far proven resilient to climate change and have been largely immune from threats to other reefs worldwidePhotograph: Anne and Charles Sheppard/Chagos Conservation Trust

But the Chagos Conservation Trust, a member of the CEN, says legal and illegal fishing has impacted the area despite regulations, with sharks, sea cucumbers, turtles and fish known to have declinedPhotograph: Anne and Charles Sheppard/Chagos Conservation Trust

A masked booby colony. Over 150,000 pairs of 17 species of seabirds breed on the atolls, and protection of the islands would certainly improve their prospects Photograph: Anne and Charles Sheppard/Chagos Conservation Trust

A lack of inhabitants means the islands are rat-free, and full of nesting birdsPhotograph: Anne and Charles Sheppard/Chagos Conservation Trust

Ground nesting birds are vulnerable in most places. Here they can nest and raise their young in safety

Diego Garcia atoll. If done in the right way, the Chagos protected area could be as important as the reserves which protect the Galapagos islands and Great Barrier ReefPhotograph: John Parker/Corbis