A French water charity sounded the alarm on Monday about pollution in the Seine just over 100 days before the start of the Olympics when the river is set to be used for swimming events.

Surfrider Foundation said it had analysed six months of tests over winter undertaken by a laboratory and had concluded that the river water remained polluted and potentially dangerous.

In an open letter, the Biarritz-based charity said it “wanted to share with stakeholders its rising concerns about the quality of the Seine but also the risks faced by athletes moving in contaminated water.”

Paris authorities are in a race against time to clean up the Seine before the start of the Olympics on July 26, with the famed waterway set to play a starring role during the Games.

It is expected to host the opening ceremony, and will then be used for the marathon swimming events and the triathlon — pollution permitting.

Cleaning up the Seine has been promoted as one the key legacy achievements of the Paris 2024 Games, with mayor Anne Hidalgo intending to create three public bathing areas in the river next year.

She and President Emmanuel Macron have also promised to take a dip before the Games to demonstrate it is safe — just over a century since public swimming was banned there in 1923.

Around 1.4 billion euros ($1.5 billion) has been spent upgrading sewage and storm water treatment facilities to reduce the amount of untreated faecal matter flowing into the river and its main tributary, the Marne.

Marc Guillaume, the top state security official for Paris, attacked Surfrider’s “very poor knowledge of this issue”, adding: “There’s no point doing tests today in the Seine and comparing them with what will happen next summer.”

Surfrider said the tests had been carried out by the laboratory Eau de Paris and environmental analysis group Analy-Co from September to March underneath the bridges Alexandre-III and l’Alma, where the Olympic swimming is set to take place.

European water quality standards and the international triathlon and open-water swimming federations set limits on the concentration of two bacteria — E. Coli and enterococci — which are indicators of the presence of faecal matter.

Out of 14 rounds of testing, only one had shown “satisfactory” results, Surfrider said.

Overall, the testing had shown “alarming” levels often double and sometimes three times higher than the maximum permitted amounts. One reading showed E.Coli at seven times the maximum level.

Paris authorities stressed that the samples were taken over the winter period — and one of the wettest winters in 30 years.

Heavy rainfall is known to overwhelm Paris’ more than century-old sewage system, leading to direct discharges into the river of untreated effluent.

“The first quarter of 2024 saw extremely large rainfall (250 000 ml over three months, double the level of 2023) which deteriorated the quality of the water,” the Paris prefect’s office said in a statement.

Furthermore, water disinfection facilities were not working over winter and would be started ahead of the Olympics.

“There has never been question of opening the Seine for swimming all year round,” Guillaume added.

A giant new underground storm water facility is expected to be inaugurated later this month in southeast Paris, while the last sewage connections for river boats in the centre of the capital are currently being completed.

Cleaning up the Seine had been “probably the most difficult (Olympic) project to organise,” deputy Paris mayor Emmanuel Gregoire told reporters last Friday.

Three Olympic test events in the Seine were cancelled last July and August due to elevated E. Coli readings.

The disruption was blamed on very heavy rainfall and a faulty sewer valve which led to dirty water being discharged into the river.

Last month, Brazilian swimmer Ana Marcela Cunha, the reigning Olympic open water champion, called on Paris to have a plan B in case the Seine was not ready.

Organisers say they are able to delay the swimming events in the event of heavy rainfall, but they do not intend to move them.

Olympic open-water swimming has been hit by pollution concerns in the past.

At the end of the test event in 2019 ahead of the Tokyo Olympics, swimmers protested against the quality of the water in Tokyo Bay.

At the Rio Olympics in 2016, the prospect of swimming in the polluted Guanabara Bay also made headlines.