The Scottish beach where Donald Trump is planning “the greatest golf course in the world” has fallen out of favour with a respected environmental group, which is no longer recommending it to bathers.

The sparkling water at Balmedie may have seduced the American billionaire but, according to the Marine Conservation Society (MCS), it is somewhat murkier below the surface. In the society's latest Good Beach Guide, the Aberdeenshire spot has lost its “recommended” rating and dropped down two categories to only a “basic pass” - the EU statutory minimum standard for water quality.

Calum Duncan, MCS Scottish Conservation Manager, said that there were sewer overflows at the beach and the heavy rain of last summer could have caused them to run over and pollute the sea. He said that the bad weather would also explain a decline in standards across much of Britain.

George Sorial, who is leading the Balmedie development for the Trump Organisation, said that the company would be happy to offer its own environmental experts' advice on “how to remedy the situation”. He said that it was “in everyone's interests to keep the water pristine”.

“Like any piece of land under urban pressure, it needs to be properly studied and managed, in order to preserve it for future generations,” he said. “All the rhetoric aside, to date we are the only group that studied and surveyed and proposed a plan to properly manage the site and preserve it.”

Balmedie is not by any means the grubbiest of British beaches. The Cornish resort of Rock, dubbed Chelsea-on-Sea after Princes William and Harry began frequenting its beaches and bars, has been exposed as having some of the dirtiest sea water in the country.

The beach at Rock, where bungalows sell for £3million, is one of 78 in Britain that have failed to reach the European Commission's mandatory standard for water quality.

Almost a sixth of the beaches in the 2009 Good Beach Guide have lost their recommendation for excellent water quality - the biggest year-on-year fall in the 22 years that the MCS has produced the guide.

For the first time since 2002 the number of recommended beaches is below 50 per cent of those tested. Water samples were taken from 777 British beaches, but only 370 were recommended this year for swimming, surfing or paddling.Of the 109 Scottish beaches tested, 17 failed and 29 performed worse than last year. There were 41 recommended for bathing - 15 fewer than the best showing, in 2004.

The testing, between June and September, coincided with one of the wettest summers on record in Scotland. According to the MCS, flood water and sewage gushed from sewer overflows, and polluted storm water ran off farmland and city streets into rivers and the sea. North and South Ayrshire, East Lothian, St Andrews and the Edinburgh city beaches of Portobello were badly affected.

Overall, Scotland's beaches have come thorugh the storms slightly better than the rest of Britain. There are 41 places north of the Border that are recommended for bathing - a decline of 7 per cent. Across Britain, there was a drop of 16.5 per cent to 370 recommended beaches - the biggest fall in the 22-year history of the survey and the first time since 2002 that less than half of Britain's beaches made the category.

The best Scottish beaches can be found in the Highlands, according to the guide, which recommends 16 different locations in the region.

Three different beaches at Gairloch, on the west coast, are mentioned. They sit south of Camusdarach, near Arisaig, which received international recognition when the actor Ewan McGregor deemed it the best beach in the world.

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