The Surfrider Foundation said on Monday that it had analysed laboratory tests and concluded that the water of the Seine, which stretches almost 800 kilometres across France, remains contaminated and potentially dangerous in the Paris area, where Olympic swimming trials are due to take place.

In an open letter, the Biarritz-based foundation said it wanted to "share with stakeholders its growing concern about the quality of the Seine, but also the risks athletes face when swimming in contaminated water". Paris authorities are in a race against time to clean up the Seine before the Olympic Games begin on 26 July, with the famous river set to play a starring role in the marathon swimming and triathlon events during the Games, if pollution is allowed.

The opening ceremony will be held on France's most famous river, which won't pose any major problems as the 180 boats carrying athletes won't be swimming or coming into contact with the water, which is still polluted despite government efforts to clean it up for the capital's third Summer Games.

It is estimated that around €1.4 billion has been spent in the Paris region over the last decade on upgrading wastewater and stormwater treatment facilities to improve the quality of the Seine and its main tributary, the Marne. A major new raiwater treatment plant is due to be inaugurated this month, while new sewage connections for river boats continue to be built.

Surfrider said the tests were carried out by the Eau de Paris laboratory and the environmental analysis group Analy-Co between September and March under the Alexandre-III and l'Alma bridges, where Olympic sports will be held.

European water quality standards and international triathlon and open water swimming federations set limits for the concentration of two dangerous bacteria - Escherichia coli and enterococci - which come from faeces and can cause various infections, including haemolytic uremic syndrome (HUS), a condition that can lead to kidney failure, bloody diarrhoea and even death (common in children under five in Africa).

Surfrider said their measurements showed levels often double and sometimes triple the maximum permitted levels. The main hope of the organisers and the Paris authorities is that these levels, recorded during the winter and one of the wettest winters in 30 years, will subside in the summer.

The rain overwhelms the Paris sewerage system, leading to the direct discharge of untreated sewage containing bacteria derived from flora. The organisers have always maintained that Olympic sports can only be held in the river if the weather is dry or if it rains.

Cleaning up the Seine is set to be a key legacy of the Paris 2024 Olympics, with Mayor Anne Hidalgo promising to create three public bathing areas in the river next year. She, and President Emmanuel Macron, have pledged to swim there to show it's safe. Olympic open water swimming has often been marred by pollution concerns, both in Tokyo Bay and Rio de Janeiro's Guanabara Bay.