The heavy rain last summer has been blamed for a 10% drop in the number of Scottish beaches awarded the highest rating for water quality, the Marine Conservation Society (MCS) said yesterday.

The watchdog blamed last year's downpours for washing pollutants into the sea and causing 18 beaches to fail minimum European standards - up from seven the year before.

While acknowledging that last year's bathing water survey results represented a blip against a long-term improvement in quality, MCS has expressed concern that storm-related pollution is set to increase and required further investment in Scotland's water infrastructure.

Calum Duncan, MCS Scottish conservation manager, said: "These latest results buck the long-term trend toward cleaner bathing water but we're pinning the blame on last summer's exceptionally bad weather. Heavy rain sweeps pollutants such as 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."

Out of 108 beaches tested by MCS, 44 were "recommended" by the society.

The testing period, from June to September 2007, coincided with the wettest summer in Scotland for 30 years.

Seven of the 18 beaches that failed European standards were in Ayrshire, which is prone to agriculture-related pollution occurring when rainwater washes animal waste off the land. They include Prestwick and Ayr South, which went from passing basic standards in 2006 to failing them.

A further two beaches failed in Aberdeenshire, one each in Moray and Angus, three in Fife and one in East Lothian and the Borders.

In the Highlands, despite its massive coastal area, only two beaches failed the basic safety requirements - Portmahomack in Ross-shire and Gruinard Beach in Inverness-shire.

Across the UK, the number of beaches failing the minimum European legal standards for water quality more than tripled, from 17 to 53.

The fall in recommended beaches represents the biggest year-on-year drop receiving the top rating in the guide's 21-year history, with 443 UK beaches recommended this year compared to 495 last year.

Scotland has long suffered from some of the most polluted beaches in Europe and tough regulations due to be imposed in the next few years are set to lead to more failures.

However, investment of £3bn in sewage treatment and infrastructure over the past seven years is credited in cleaning up many of Scotland's beaches - the 44 recommended this year is triple the total figure in the Good Beach Guide edition of 1998.

Mr Duncan praised the approach of the Scottish Government and Scottish Environment Protection Agency (Sepa) in improving bathing water quality but said that storm run-off still threatened to hamper their efforts.

"Electronic forecasting signs advising swimmers about water quality have been a great success and we need more of them, but we also now need serious investment to tackle pollution from the hundreds of storm overflows around the coast and the more than two dozen outfalls continuously pumping poorly treated sewage into the sea," he said.

"MCS is very supportive of the strategy and action plans adopted by Sepa and the Scottish Government to tackle coastal pollution but we equally have concerns that Sepa has reduced bathing water testing by more than 30% since 2003."

Calum McPhail, Sepa's environmental quality manager, said: "Sepa considers climate change as a factor when planning any new sustainable water management infrastructure such as Sustainable Urban Drainage Systems and better drainage capacity as well as currently maintaining 10 electronic signs used for early warning forecast and prediction of the bathing water quality at beaches."