'Below acceptable standards': Olympic open water test in Seine postponed
"The decision has been made, in consultation with public health authorities and event organising partners, to postpone the women's race originally scheduled for August 5," the FFN said in a statement.
"The quality of the water in the Seine currently remains below acceptable standards for safeguarding the health of swimmers."
Initially, the women were to compete on Saturday and the men on Sunday.
They will now compete successively Sunday at 0530GMT for women and at 0930GMT for men over a 10km loop between the Pont Alexandre III and the Pont de l'Alma, the same route as the 2024 Olympic event.
Persistent heavy rain in Paris had lead to concerns over pollution from the overflow of sewage.
Friday morning's training session had already been cancelled, and organisers will again analyse water samples before Sunday's competition.
"Water quality continues to be closely monitored," the FFN said.
"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 FFN spokesperson said after deciding along with World Aquatics and French health authorities to scrap Friday's practice.
Deadly bacteria
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 Paris region has suffered the heaviest summer rainfall recorded over the past 20 years," it added.
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 made headlines.