The future of the Great Barrier Reef may depend on a type of 'cling film' 50,000 times thinner than a human hair.

Scientists have created the biodegradable covering to block the sun's rays and protect the reef from bleaching.

This occurs when corals, under heat stress, expel the algae living in their tissues, turning completely white. David Attenborough has warned such reefs could die off by 2050 because they rely on algae for food and are more vulnerable to disease after bleaching.

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In trials the film reduced the light reaching seven different coral species by up to 30 per cent.

Anna Marsden, managing director of the Great Barrier Reef Foundation, which helped to develop it, said: 'The sun shield is 50,000 times thinner than a human hair and completely biodegradable, containing the same ingredient corals use to make their hard skeletons – calcium carbonate.

'It's designed to sit on the surface of the water above the corals, rather than directly on the corals, to provide an effective barrier against the sun.'

In early 2016 the Great Barrier Reef suffered the worst bleaching yet, affecting an estimated 93 per cent of coral on its vast northern section and killing more than a fifth.

Last year it happened again, raising fears for the future of the 1,400-mile long World Heritage site.

Carbon dioxide pollution is believed to make the problem worse, by trapping heat within the atmosphere. The trials found the ultra-thin film decreased bleaching of most coral species. It is understood that it can join itself back together if disturbed by divers or fish. Miss Marsden said it was impractical to suggest the 'sun shield' could cover the entire reef.

The ultra-fine biodegradable film some 50,000 times thinner than a human hair. Pictured are live coral in sun shield trials in Townsville, northern Queensland

A fifth of the Great Barrier Reef died off in 2016 after significant bleaching

'But it could be deployed on a smaller, local level to protect high-value or high-risk areas of reef,' she added. 'The concept needs more work and testing before it gets to that stage, but it's an exciting development at a time when we need to explore all possible options to ensure we have a Great Barrier Reef for future generations.'

The reef is also under threat from plastic pollution and run-off of waste from farms. It is home to 3,000 varieties of molluscs, more than 100 types of jellyfish, 1,625 species of fish, 133 types of sharks and rays and more than 30 species of whales and dolphins.

The film was developed with help from the University of Melbourne and the Australian Institute of Marine Science.