And they estimate that more than half the reefs are under threat from human activities.

The UK is responsible for many reefs by virtue of having overseas territories. The top image on this page shows the Caicos Bank, the larger part of the British Overseas Territory of the Turks and Caicos Islands, southeast of the Bahamas. The tail of the space shuttle can be seen in this image.

Reefs have become a major source of income for the tourist industry. Divers are drawn to the reefs to photograph their diverse lifeforms, like (above right) schooling bannerfish, a type of butterflyfish, often seen as the emblem of healthy reefs.

The coral atoll of Diego Garcia (right) is just a small part of the British Indian Ocean Territory of Chagos Archipelago.

Indonesia has most reefs, followed by Australia and the Philippines.

France is fourth, and the UK twelfth, ahead of the US because of those overseas territories.

The Coral Reef Alliance underlined the importance of maintaining healthy reefs. It said several important drugs had already been developed from chemicals found in coral reef organisms. The most famous of these is AZT, a treatment for people with HIV infections, which is based on chemicals extracted from a Caribbean reef sponge

A fisherman trawls the shallow waters around Antigua.

Small-scale fisheries on reefs provide food and employment for hundreds of millions of people world-wide, but the WCMC report says growing populations and over-utilisation threaten the long-term survival of the resources on which many people depend.

The spiny lobster (right) has become a popular dish on many tourists' plates.

The researchers say these animals can be harvested sustainably if a few simple management measures are enforced.

Images courtesy of Unep-WCMC (Mark Spalding) and the Earth Sciences and Image Analysis Laboratory, Nasa, Johnson Space Center.