"The rules of the International Triathlon Federation allow the event to be switched to a duathlon format as a last resort. Conversely, in order to guarantee that the marathon swimming events would take place, once our options for postponement had been exhausted, we decided to set up a reserve site at Vaires-sur-Marne," a Paris 2024 spokesperson told Reuters.

"The competition site, already used for rowing and canoeing events, has all the necessary features to host these events if required."

The quality of the Seine water has improved, data showed on Thursday, three weeks before the Olympics.

Data published by the city and regional authorities showed the concentrations of enterococci and E.coli bacteria were below legal thresholds six out of nine days between June 24-July 2.

The levels depend on rainfall and water temperature among other factors. With decent weather this week in Paris, the quality of the water is expected to remain good.

The French capital has been working on cleaning up the Seine so people can swim in it again, as was the case during the 1900 Paris Olympics. But a sewer problem last summer led to the cancellation of a pre-Olympics swimming event.

The swimming leg of the triathlon and the marathon swimming event of the Olympics, which run from July 26 to Aug. 11, are scheduled to be held in the Seine.

Paris Mayor Anne Hidalgo, who championed a campaign to clean up the once infamously dirty river in time for the Olympics, last month postponed her planned dip in the river.

She said then it was likely to happen after the French snap elections, whose second round is due to take place on Sunday, and after the annual July 14 Bastille Day celebrations.

"We have every confidence in the work undertaken by the State, the City of Paris and all the parties involved to make the Seine swimmable so that the events can take place there as planned," the spokesperson added.