Olympic organisers cancelled a second day of triathlon training in the River Seine planned for Monday after weekend downpours in Paris polluted the waterway, but said they were "confident" the medal events would be held this week.
In a joint statement, Paris 2024 organisers and World Triathlon said they had taken the decision to cancel Monday's swimming training session because "water quality levels... do not present sufficient guarantees" to allow it to take place.
The triathlon is the first Olympic event due to be held in the Seine, before marathon swimming in the second week of the Games. A swimming training session slated for Sunday was also cancelled due to the pollution levels.
The men's individual triathlon is scheduled to start on Tuesday at 8am, with the women's individual event to be held on Wednesday.
The quality of the Seine's water is dependent on the amount of rain that falls in and around Paris.
Heavy downpours of the sort seen on Friday during the sodden opening ceremony of the Games overwhelm the city's sewage system, leading to discharges of untreated effluent into the waterway.
The Paris 2024 organisers and World Triathlon, however, said they were "confident" that water quality would improve sufficiently before the start of competition on Tuesday, taking into account the weather forecast for the next 48 hours.
After downpours on Friday and Saturday, the sun has returned to the French capital since Sunday morning.
Due to an exceptionally wet spring and start to summer, the Seine had been consistently failing water tests until the start of July.
Levels of the E.Coli bacteria – an indicator of faecal matter – were sometimes 10 times higher than authorised limits.
Paris 2024 organisers have the ability to delay the outdoor swimming events by several days in the event of rain.
As a last resort, they have said they will cancel the swimming leg of the triathlon and move the marathon swimming to Vaires-sur-Marne, on the Marne river east of Paris.
French authorities have invested €1.4 billion ($1.5 billion) over the last decade to clean up the Seine.
On July 17, Paris Mayor Anne Hidalgo swam in the Seine along with Paris 2024 chief organiser Tony Estanguet to demonstrate it was ready for the Olympics.