The men's individual triathlon has been postponed after tests revealed insufficient water quality levels in the River Seine.
The race was due to start at 07:00 BST (08:00 local time) on Tuesday and will now take place on Wednesday at 09:45 (10:45 local time), following the conclusion of the women's event.
Swimming training sessions were cancelled on Sunday and Monday because of the water quality, which was impacted by recent heavy rainfall.
Friday, 2 August remains a contingency date for both races, while organisers have said the event could be contested as a duathlon as a last resort.
"Tests carried out in the Seine today revealed water quality did not provide sufficient guarantees to allow the event to be held," World Triathlon said.
"Despite the improvement in the water quality levels, the readings at some points of the swim course are still above the acceptable limits."
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.
Tests are being carried out daily on the water quality in the River Seine, which is also due to host the triathlon mixed relay on 5 August, the Olympic marathon swimming on 8 and 9 August, and the Para-triathlon event at the Paralympics, which start on 28 August.
