Tests carried out on Saturday “revealed water quality levels that in the view of the international federation, World Triathlon, did not provide sufficient guarantees to allow the event to be held”.
Sunday’s training session for the Olympic triathlon event in Paris has been cancelled due to unsafe levels of pollution in the River Seine.
Triathlon events are due to start on Tuesday with the men’s race.
But tests carried out on Saturday “revealed water quality levels that in the view of the international federation, World Triathlon, did not provide sufficient guarantees to allow the event to be held”, Paris 2024 said in a statement.
The heightened pollution levels are the result of the heavy rain that blighted the Olympic opening ceremony on Friday and initial outdoor events on Saturday, they added.
Preparations for the running and cycling sections of the triathlon are going ahead as planned on Sunday.
Organisers say they are “confident” water levels will return to normal to allow triathletes to swim in the water on Tuesday.
Experts are convening at around 4am each morning to assess water quality levels.
Paris officials have spent €1.4bn in a bid to improve water quality ahead of the Games.
This includes the installation of a 46,000 cubic metre storage basin near the Gare d’Austerlitz train station on the Seine’s left bank.
The basin holds the equivalent of 20 Olympic pools’ worth of rain and wastewater, which helps reduce toxicity levels seeping into the river.
Seine swimming banned in 1923
Parisians were banned from swimming in the Seine a century ago, with Olympic athletes last allowed to do so at the first Paris Olympic Games in 1900.
Paris mayor Anne Hidalgo took a dip in the water earlier this month to prove the water is clean enough.
She had originally planned to swim in the river last month but was forced to delay after tests indicated the presence of faecal matter 10 times higher than authorised limits.
Ms Hidalgo was joined by Paris 2024 chief Tony Estanguet but there was no sign of President Emmanuel Macron who had suggested he would also take the plunge.