An average of 2,195 pieces were found for every kilometer cleaned in the society's Beachwatch project last year, more than two items per meter and more than double the amount found in the first clean-up in 1994.
The amount of plastic rubbish found had tripled in the same time and made up about 60 per cent of the waste recovered, said Emma Snowdon, the project's coordinator.
South West England was the worst affected area, with almost 5,000 pieces of litter on every kilometer of beach. Almost 40 per cent of the litter was discarded by beach users.
Litter is often mistaken for food by marine wildlife, and this can cause starvation, poisoning and fatal stomach blockages.
Plastic waste breaks into ever-smaller fragments but does not disappear. Even beaches that seem clean have up to 5,000 plastic fibers per liter of sand. Pellets of plastic can concentrate toxins from surrounding seawater. Once consumed by fish, the pollutants enter the food chain.
More than 5,000 volunteers took part in the clean-up operation in September. Although 374 beaches were cleaned, their 175 kilometers make up less than 1 per cent of the coastline.
Ms Snowdon said: "As a small charity, sorting out this huge problem is beyond us. Though every member of the public can help by reducing, reusing and recycling, there are too many other sources of waste involved."
She called for an official marine litter strategy to combat the problem.
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