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Unsafe pollution levels in the River Seine forced the postponement of the men's triathlon at the Paris Olympics on Tuesday.
In a massive blow to Olympic organisers who have repeatedly vowed that the Seine would be safe to swim in, Tuesday's race was called off just hours before the start after last-minute water quality tests.
A joint statement from Paris 2024 and World Triathlon blamed the spike in pollution levels on the heavy rain that fell in the French capital on Friday and Saturday.
The men's triathlon has now been rescheduled to take place on Wednesday, immediately after the women's event.
However, the forecast of a major storm with more heavy rainfall hitting Paris later on Tuesday has raised serious doubts over whether Wednesday's rescheduled events could go ahead.
Heavy downpours still overwhelm the city's underground drains and sewage system, leading to untreated effluent being released into the waterway.
If water quality does not improve in time, the swimming leg of the event would be removed and the race converted to a duathlon.
World Triathlon president Marisol Casado said she was still hopeful that the races could take place on Wednesday.
"We are all disappointed because we are not able to follow strictly what was planned," she told a press briefing.
"At this point, we are quite confident that we will have them [races] tomorrow and then we will be very happy all of us, the first ones the athletes."
Paris 2024 sports director Aurelie Merle added: "We are quite hopeful because you can see the weather conditions have been quite good over the past few days with the return of the sun and the high temperatures, which have a positive impact on the water quality."
Benjamin Maze, the technical director of the French Triathlon Federation, told FranceInfo radio that there was "a bit of anger" among competitors who had risen early only to learn of the postponement.
"The athletes got up a bit before 4am, they were having breakfast when we received the information from the international federation," he said.
"Obviously there's a bit of anger and a lot of disappointment."
The last race to be turned into a duathlon because of water problems was the Europe Triathlon Championship in Madrid last year.
Tuesday's weather forecast looked ominious.
National forecaster Meteo France warned about the danger of strong thunderstorms, heavy rain, hail and lightning from 6pm Paris time over the French capital which could be accompanied by "intense precipitation".
Around 25mm of rain fell on Friday and Saturday, the equivalent of the average rainfall for the whole of July, according to Merle.
The Seine is set to be used for the triathlon mixed relay on August 5 and then again for marathon swimming – a 10-kilometre swim in open water – on August 8-9.
Marathon swimming can be moved to another location in Vaires-sur-Marne on the River Marne east of Paris if necessary.
Paris Mayor Anne Hidalgo swam in the Seine earlier this month along with Paris 2024 chief organiser Tony Estanguet to demonstrate it was clean enough for the Olympics.
The Socialist city leader plans to create three public bathing areas in the Seine for Parisians next year – a century after swimming was banned in the river.
In the women's event, Great Britain's Georgia Taylor-Brown, who claimed individual silver and a gold medal in the mixed team relay at the Tokyo Games, has said she is just happy to have made the starting line this time after her injury struggles.
