Mass releases of balloons are posing a deadly threat to wildlife and should be banned, marine conservationists said today.

The practise of releasing thousands of balloons into the sky has become popular with charities and at commemoration events. But the debris left behind is killing seabirds, turtles and even whales and dolphins, according to the Marine Conservation Society, a charity. It says there has been a threefold increase in the number of balloons and balloon pieces found on British beaches since 1996.

"MCS has evidence of the potentially lethal effects of balloons on animals," said Gill Bell, a spokesman for the charity. "A balloon release is not a cause for celebration but a possible death sentence for dolphins, turtles, whales, sharks, fish and seabirds."

Birds have been known to have drowned after becoming trapped in balloons, and others to have starved after swallowed balloon fragments blocked their digestive systems, MCS says. Turtles have also been found dead with balloons in their guts.

Ms Bell says the scale of the problem is difficult to judge, as there have been no systematic studies, "but we know that animals have been killed and numbers of balloons have increased. We think this is the tip of the iceberg." Balloons are usually in the top five causes of death for birds washed ashore on beaches, she said.

A study of beaches by the MCS last year found that the number of balloons and balloon pieces found on beaches had rised from from 3.4 items per kilometre to 11.5 items per kilometre. A US study in 2004 found 63,000 balloons on beaches worldwide.

The charity is calling on the Government to class the practise as littering under the Environmental Protection Act. "Balloon releases are littering. Whatever floats comes back down".

Although many balloons are biodegradable, Ms Bell says "it could come into contact with an animal in the meantime".

People considering a balloon release should think again, Ms she says. "Most people wouldn't want to do one if they knew the result".

However, Nabas, the balloon association, denied that balloons were a threat to wildlife. "Balloon releases conducted by professional balloon artists are not harmful to the environment," it said in a statement.

"All balloon releases should adhere to our Code of Conduct which was drawn up with the Marine Conservation Society a few years ago. This Code advises that balloons are released without strings, clips, discs, ribbons, or any other objects attached. Released correctly and responsibly under these guidelines, balloons will rise about 5 miles into the atmosphere where they become brittle and shatter into tiny fragments. During a UK beach clean and survey report in 2006, balloons represented only 0.5 per cent of the litter collected."

Alternatives suggested by the MCS include balloon art, "guess the number of balloons" competitions, and prize balloon-popping.

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