Sewage overflow pipes are posing a growing threat to the water quality in some of Scotland's beaches, environmental campaigners have warned.

The Marine Conservation Society (MCS) said that hundreds of overflow pipes - which are used to ease the pressure on sewers during periods of heavy rain - represented a "licence to pollute" the coastline with raw sewage.

Details of 3,500 overflow pipes operated by Britain's water companies emerged in a report yesterday.

Thomas Bell, a coastal pollution officer for the Marine Conservation Society (MCS), said: "There is no limit on the amount of sewage that can come out of these overflows. They represent a licence to pollute."

The MCS obtained details of the locations of the sewer pipes under freedom of information legislation.

They reveal that nearly 300 overflow pipes are operating in Scotland, where several beaches have failed water quality tests.

Among Scotland's worst affected beaches are Lower Largo in Fife and Stevenston in North Ayrshire, which both have sewer overflows. They have failed to reach the legal minimum EU water quality standard for the last two years.

East beach in Lossiemouth, Moray, and Easthaven in Angus have seen a dramatic decline in water quality standards over the past year. Both were MCS recommended beaches in 2006 for having the highest quality water standard and good treatment of sewage discharges. However, they both failed water quality tests last year. Easthaven has two sewer overflows while Lossiemouth has nine.

MCS has identified more than 50 beaches, 13 of them in Scotland, which have sewage overflow outlets and where water quality declined in 2007, compared to the previous year.

Among others which have been downgraded are the Prestwick, Ayr South and Girvan beaches in South Ayrshire. They all failed to meet the minimum standards last year, having achieved basic passes in 2006 - and all have several sewage overflows.

The charity said that around the UK, about 3,500 combined sewer overflows - which act as a flood-release mechanism for sewers carrying both sewage and storm water - are in urgent need of review.

Mr Bell said: "The problem is that the sewer system was designed for a climate which is different from the one we have got now. Because of the effects of climate change we are having a lot more rain. The combined sewer overflows are starting to discharge a lot more often than they should.

"It amounts to a licence to pollute, because there are no restrictions on how much and how often these pipes can discharge."

Scottish Water said it was working with the Scottish Environment Protection Agency to reduce pollution from overflows. A spokesman said: "In the first four years of its existence, Scottish Water made significant improvements in both quality of tap water and in cleaning up coastal and river environments.

"We are now into the next four-year regulatory period, which will see us invest £2.4 billion up to 2010 - the second biggest investment programme in the UK. In 2006-7 a total of £17.4 million was invested to improve the environment.

"European legislation has set high standards for protecting the environment especially with regard to the discharge of waste water into rivers and coastal waters. Scottish Water is totally committed to this cause."