The 46-year-old was accompanied by para-triathlete Alexis Hanquinquant, who will be host nation France’s flag-bearer at the Paralympics opening ceremony on August 28.

Weather permitting, the river will be the star of the opening ceremony of the Olympics on July 26 and will then host the triathlon and the open water swimming.

Oudea-Castera, dressed in a body suit, slipped as she entered the Seine, but stole the thunder from Anne Hidalgo, the mayor of Paris, with whom she has notoriously poor relations.

Hidalgo had flagged up that she would swim on July 17 if the water quality was good enough.

Oudea-Castera seized her opportunity — exclusively filmed by news channel BFMTV — a day after Paris city hall said the Seine has been clean enough to swim in for most of the past 12 days.

The quality of the water met the required standard for “11 days or 10 days” of the past 12, city hall official Pierre Rabadan told broadcaster RFI.

The Paris region has seen an unseasonably heavy amount of rain over recent weeks, which has raised the Seine’s pollution levels as untreated sewage is washed into the river.

“We hope the weather will get a little better, but we are not worried about the possibility of holding the competitions,” Rabadan said. “They will take place.”

He added, however, that there may have to be “modifications”, without giving details.

Weather in Paris is forecast to be mostly dry over the final 14 days before the start of the Games.