Conservationists say the Australian government must undertake a multi-million dollar cleanup of the Great Barrier Reef or risk its destruction. Campaigners say the World Heritage Site is threatened by millions of tons of chemicals and mud washed from farms onto the reef each year. From Sydney, Phil Mercer reports.

Environmentalists say the Great Barrier Reef is being threatened by waves of dangerous agricultural sludge.

Campaigners believe that more than 90 percent of the reef's pollution comes from soil, pesticides and other chemicals that have washed off farms and sugarcane plantations into rivers.

There are concerns that nutrients from farmland runoff boost the population of the crown-of-thorns starfish. It is a voracious predator that attacks coral.

The Australian government is being urged to do more to help farmers reduce this run-off.

Nick Heath from the global environmental group WWF says urgent action is needed.

"If nothing is done it's quite a grim future for the reef," he said. "Pollution will continue to stress corals, continue to feed wave after wave of crown of thorn starfish outbreaks, reducing coral cover and probably even worse reducing the resilience of the reef to be able to deal with the increased temperatures expected from climate change."

The Great Barrier Reef is home to 400 species of coral and supports over 2,000 different types of fish and 4,000 species of mollusks and countless other invertebrates.

It is the world's largest coral system and stretches for 2,300 kilometers along Australia's northeast coast, covering an area bigger than Britain.

The reef is one of Australia's most important tourist destinations, contributing an estimated $5 billion to the economy.

It attracts a million visitors a year, who come to see a vast array of sharks, turtles and brightly colored fish.

A new study by researchers at the University of North Carolina in the United States has found that the degradation of the world's coral reefs has been much worse than previously thought. A report said that over the past 20 years, coral had disappeared at five times the rate of the world's rainforests.

Many scientists have blamed climate change for rising ocean temperatures, which can kill coral by causing what is called bleaching.