Swedish marathon swimmer Victor Johansson has pulled out of the Olympic men's 10km event scheduled for Friday citing concerns about the water quality of the Seine river, Swedish newspaper Dagens Nyheter reported.
Both Paris 2024 organisers and World Aquatics guaranteed the water quality met the thresholds set by the sport's governing body.
Ireland's Daniel Wiffen, who has already won gold and bronze medals in the pool, is due to take part in Friday's marathone event in the river.
Johansson told the paper triathletes had fallen sick after swimming in the urban river during the Olympic competition, without naming any.
"There is a lot of information that has been flying around, but what we know for sure is that people have become ill," the paper quoted him as saying.
"So even though the levels (of E.coli bacteria) have gone down, it didn't feel good to start.
"The triathletes were in the Seine for about 20 minutes, and despite the short time, some got sick," Johansson added.
"Paris 2024, together with World Aquatics, has put in place a rigorous water quality monitoring process," Games organisers said in a statement to Reuters.
"Today’s results were considered 'very good' according to the World Aquatics thresholds for E.coli and Enterococci on all four testing points across the marathon swimming course."
World Aquatics in a statement added: "The men's marathon swim will only take place after analysis of the latest water quality readings overnight.
"We are very confident that the water quality will remain very good and that the race will be able to go ahead."
Swedish media reported that the nation's triathlete Tilda Mansson fell sick and was vomiting a few days after competing in the women's event on July 31, though the team did not confirm there was a connection with the Seine water.
"We have to be in the water for two hours," said 25-year-old Johansson.
"You swallow anywhere from 0 to 250 millilitres (8.45 fluid ounces) of water per hour, so at worst I would have been able to come up from the Seine with half litre of water."
Swedish Olympic Committee doctor Lykke Tamm said Johansson had dialogue with the medical team which backed his decision not to swim.
"We fully support Victor's decision not to participate," Tamm said in a statement.
"After careful consideration of all the factors surrounding Victor, my recommendation is that he should not swim in the Seine as it stands now. Health is always most important."
The Netherlands' Sharon van Rouwendaal won the women's 10km event earlier on Thursday, saying she had swallowed some of the Seine water during her swim but was fine about an hour after the race.
The women's event went ahead as scheduled after organisers said the water quality had met acceptable thresholds.
French authorities spent €1.4 billion on upgrading the city's sewage systems, promising the river would be clean for residents to swim in by next summer.
However, water quality issues proved a headache during the triathlon events, with familiarisation sessions cancelled and the men's race postponed by 24 hours.
