Persistent heavy rain in Paris has threatened the pre-Olympic swimming open water test events in the Seine this weekend with concerns over pollution from the overflow of sewage.

Unseasonal downpours have seen nearly 104mm of rain falling on the French capital in the last three weeks, sometimes in tropical storm mode, resulting in the cancellation of Friday's training session.

"Following recent heavy rains in Paris, the quality of the water in the Seine is currently below acceptable standards for safeguarding the health of swimmers," a spokesperson for the French Swimming Federation (FFN) said after deciding along with World Aquatics and French health authorities to scrap Friday's practice session. 

The World Cup events, Saturday for men and Sunday for women, in a 10km loop between the Pont Alexandre III and the Pont de l'Alma, are above all a test for the Paris Olympics in a year's time.

But summer rains wash dirt from paths and roads into the water with the overflow of sewage causing bacterial pollution in particular from the potentially-deadly E.coli, which can also cause serious intestinal infections.

Based on 42 samples from June and July, French authorities had given the green light "in principle" for this weekend's competition and the swimming leg of the triathlon from August 17-20.

On Friday, Paris Mayor Anne Hidalgo's office insisted that "one year from the Games, the sanitation dynamic continues with the completion of the most significant work to improve water quality in the coming months, in particular to deal with these exceptional weather conditions". 

The events are also a prelude to the future return of swimming in the Seine promised by Hidalgo from 2025 on three sites where swimming has been prohibited since 1923. 

For this Olympic event, organisers have long scheduled to be able to postpone by two or three days, in the event of storms and heavy rain. 

Olympic open water swimming has frequently been hit by pollution concerns.

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 Guanabara Bay, also very polluted, also made headlines.

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