Tests are being carried out daily on the water quality in the 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.
Swimming in the river has been prohibited for over 100 years because of its high levels of pollution and the risk of disease – but French authorities have invested heavily to make the Seine swimmable as part of the Games’ legacy.
Organisers said about 1.4bn euros (£1.2bn) had been spent on a regeneration project to make the Seine safe.
Paris mayor Anne Hidalgo recently swam in the river to highlight the clean-up efforts which have been undertaken and, earlier this month, tests showed the river was clean enough for swimming.
However, heavy rain in Paris on Friday and Saturday has caused the water quality to decline.
Thundery showers are forecast for Wednesday morning, following predicted high temperatures of up to 35C on Tuesday.
France currently lags behind the European Union’s average score, external for top-quality bathing water conditions.
Between 17-23 July, data showed that the river that dissects the French capital was suitable for swimming on six days out of seven.
As recently as June, levels of E coli were 10 times more than the acceptable level imposed by sports federations.
