The number of bathing beaches failing to meet minimum health and hygiene standards has trebled because of heavy rain.

Water samples at almost 800 beaches around Britain revealed that the number failed by testers rose from 17 to 53, the worst for five years. The declining water quality is such that the number of beaches recommended for bathers by the Marine Conservation Society has slumped by 10 per cent.

Thomas Bell, the society’s coastal pollution officer, said that last summer’s record rainfall was responsible for the increase in failing beaches. More bacteria from livestock faeces were being washed into rivers and the sea from fields and farmyards.

Fertilisers, pesticides and veterinary medicines also get washed from farmland and end up in the sea where they can be a hazard for human and marine health.

Over the past decade many fewer beaches have failed tests and the society has been able to recommend more as having exceeded European standards.

Mr Bell said that the improvement was largely the result of an estimated £20 billion that has been invested in sewage treatment to prevent filthy water from being pumped into rivers and the sea.

Over the past five years notable changes in standards have been blamed on changes in the weather, with long dry periods helping to keep the quality of bathing water high and wet weather resulting in dirtier beaches. The society expects that, without measures to counteract the effects of climate change, the standard of beaches will worsen because more intense summer storms and wetter winters are forecast.

To bring about further long-term improvements in bathing quality Mr Bell says that farm management needs changes to reduce the quantity of pollutants being washed off the land. Simultaneously, sewer overflow systems need to be upgraded so that when heavy rain sweeps across the country the drainage systems can cope instead of becoming overwhelmed and diverting sewage and other polluted water into rivers and the sea.

“The trouble we’ve got now is the weather is changing more rapidly than people expected,” Mr Bell said. “The next big challenge is how we deal with the weather and the storm-related pollution.

“These latest MCS [Marine Conservation Society] results buck the long-term trend of cleaner bathing water but we’re pinning the blame squarely on last summer’s exceptionally bad weather.

“Heavy rain sweeps pollutants like raw sewage, street debris and animal waste directly from the land into rivers and the sea. Not all beaches are affected but the problem for swimmers is knowing when and where this has happened.”

After assessing samples taken from 779 bathing beaches, the society decided to recommend 443 (57 per cent) of them, a fall from 495 (63 per cent) a year ago but still more than double the number in 2000. It was, however, the biggest drop in the 21-year history of the organisation’s Good Beach Guide.

There were four grades, ranging from “recommended” to “fail” and ten beaches fell directly from the top rating to the bottom. They included the Summerleaze beach in Bude, Cornwall, Runswick Bay in North Yorkshire, and Lossiemouth in East Moray. The tests were conducted between May and September last year.

Mr Bell added: “The 443 beaches that we’re recommending shrugged off the worst of last summer’s weather, which is good.

“But poor-quality bathing water carries health risks, so if this summer is wet then I’d advise people to use the Good Beach Guide, pick bathing beaches with a good record, and stay out of the sea for at least 24 hours after heavy storms.”

The number of beaches that were recommended in England fell by 11 per cent to 278 from 312. In Scotland they fell by 10 per cent from 49 to 44, in Wales by 2 per cent from 104 to 102 and in Northern Ireland by 23 per cent from 13 to 10.

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