Pollution levels in the Seine river forced the cancellation of Saturday's test event for the para-triathlon at the Paris Olympics.
"The laboratory sample had a slightly too high level" of the E. coli bacteria, reported the Paris mayor's sports assistant Pierre Rabadan.
This follows the cancellation a fortnight ago of a test open water swim competition for the 2024 Games due to excessive pollution.
The para-triathlon test event was reduced to just cycling and running.
Sunday's mixed relay triathlon could well suffer the same fate, with a decision expected overnight, the world triathlon secretary general Antonio Arimany said.
Test events went ahead this week in the French capital's river without any issues.
Briton Beth Potter braved the waters of the Seine to win the women's triathlon and then her compatriot Alex Yee won the men's test event on a course identical to the one set to be used for next year's Games.
As organisers investigated the source of Saturday's elevated E. coli readings Tony Estanguet, the head of the Paris 2024 organising committee, stressed there was no 'Plan B' for next year's sporting extravaganza.
"There is no solution to move the event, the triathlon and open water swimming will be held in the Seine next year," he insisted.