Organisers say the men’s and women’s Olympic triathlons only have a 60% chance of going ahead on Wednesday after the men’s event was postponed on Tuesday.
Tests of the River Seine in the early hours of Tuesday morning revealed the water was not clean enough for the race to take place.
It is now set to be held on Wednesday at 09:45 BST (10:45 local time), after the women’s event, which is scheduled to start at 07:00 (08:00 local time).
Final tests of the water will be taken at 02:30 BST.
World Triathlon president Marisol Casado had previously said there was an 80% chance the men’s triathlon would take place on Tuesday.
But after the race was postponed she explained “unfortunately it was the 20%” that had won out.
When asked what the chances are of Wednesday’s triathlons happening, Casado told BBC Sport the odds are “60-40” in favour of them taking place.
“We really want this to happen. But of course we really thought that this recuperation [the improvement in the water quality] would be faster than it was,” Casado said.
“We were shocked this morning when we saw that it was not as fast as we wanted.”
The men’s race had been due to start at 07:00 BST (08:00 local time) on Tuesday.
While Friday, 2 August remains a back-up date for both the men and women’s races, organisers say the event could be contested as a duathlon – just the cycling and running legs – as a last resort.
Great Britain’s Alex Yee is one of the favourites for gold in the men’s race, while team-mate Beth Potter is the reigning women’s world champion.