Irish double medal-winning Olympian Daniel Wiffen said Monday he had "rushed" to hospital in Paris with a stomach bug, two days after taking part in the marathon swimming in the River Seine.

The 800m swimming champion and 1500m bronze medallist was meant to be carrying the Irish flag during Sunday’s closing ceremony, but did not appear.

"Thanks everyone who reached out, I’m incredibly disappointed to miss out on the opportunity to be flag bearer last night," Wiffen wrote on X in the early hours of Monday. "Yesterday I rushed to hospital as I was very unwell with a bug that I am being treated for, and am feeling better now."

Wiffen took part in the men's 10-kilometre marathon swim in the River Seine on Friday, finishing 18th with a time of 1 hour 58 minutes.

He did not link his illness to the water and organisers stress that health problems should not be automatically attributed to the river.

At least three athletes out of more than a hundred who took part in the men's and women's triathlons on July 31 reported illness after diving into the Seine during the swimming leg of the event.

They included Belgian Belgian triathlete Claire Michel who also needed medical care after suffering three days of vomiting and diarrhea due to a virus.

The Seine was dogged by pollution problems during the Olympics despite a 1.4 billion euros ($1.5 billion) upgrade to improve the Paris sewerage and water treatment system.

Due to summer rains and storms, which lead to untreated waste water being discharged into the river, the Seine failed tests on multiple occasions.

Of 11 days of events and training scheduled in its murky waters, only five got the green light.

Paris 2024 organisers insist that the two bacteria monitored by health authorities and sports federations -- E.Coli and enterococci -- were at acceptable levels when the outdoor swimming events took place.

Paris Mayor Anne Hidalgo is planning to open three public bathing spots in the river next year as part of what she sees as a key part of the Olympics legacy in the City of Light.

"If like 99 percent of the athletes don't have any problems, I think it's fine," German triathlete German Tim Hellwig told reporters after winning the mixed relay event on August 5.