The men's triathlon at the Paris Olympics will not go ahead as planned on Tuesday as pollution levels in the Seine remain too high, the sports' governing body World Triathlon said in a statement.
The race has been postponed to Wednesday at 10:45am local time (6:45pm AEST), immediately after the women's event, which is scheduled for 4pm AEST.
The decision is a blow to organisers who previously said they were confident water quality would improve with hot and sunny conditions in time for the race, after heavy rains last Friday and Saturday dirtied the river.
The 55 triathletes who were primed to compete on Tuesday now face more uncertainty.
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"Despite the improvement of water quality levels over the last hours, the readings at some points of the swim course are still above the acceptable limits," organisers said.
World Triathlon will meet with coaches on Tuesday morning to provide them with all the information and the updated schedule for the race, they said.
If bacteria levels remain too high by Wednesday morning, both the men's and women's races will likely be postponed to Friday, the contingency day reserved for the events.
For the mixed triathlon relay event next Monday 5 May, the contingency day is Tuesday 6 May.
Paris authorities have promised to make a swimmable Seine a key legacy of the Games, and spent 1.4 billion euros ($2.31 billion) on wastewater infrastructure to contain sewage and minimise spillage into the waterway.
Paris mayor Anne Hidalgo took a dip in the river herself earlier this month, in a bid to convince doubters.
The gamble that the river would be clean enough for the triathlon was never guaranteed to pay off, especially as water quality varies widely day-to-day.
Rain significantly increases concentrations of infection-causing bacteria like E. coli and enterococci.
