The first triathlon training session in the Seine was cancelled on Sunday morning due to poor water quality, leaving athletes facing an anxious wait to see if the levels improve before the men’s race is due to start on Tuesday.

Monday’s final “familiarisation” session in the Seine is also said to be touch and go, following the heavy rains that have hammered down on Paris since the opening ceremony.

Organisers have confirmed they will make a decision about that practice session at 4am local time on Monday. But if it is also cancelled it will mean that the triathletes will have no chance to practise in the Seine before they race.

However in a statement both Paris 2024 and World Triathlon insisted that the priority was “the health of the athletes”.

“Following a meeting on water quality held this morning attended by Paris 2024, representatives of World Triathlon and their technical and medical delegates, Météo France, Drieat, the City of Paris and the prefecture of the Île-de-France region involved in carrying out water quality tests, a joint decision was taken to cancel the swim leg of the triathlon familiarisation which was due to be held on 28 July at 8am,” they said in a statement. “The running and bike familiarisations will go ahead as planned.”

“Paris 2024 and World Triathlon reiterates that the priority is the health of the athletes,” it added. “The tests carried out in the Seine 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. This is due to rain that has fallen on Paris on 26 and 27 July.”

However there remains cautious optimism that the races themselves – with the women due to compete on Wednesday – will still take place on time, with organisers tending to find that water quality levels return to normal around 36 hours after the rain stops.

“Given the weather forecast for the next 48 hours, Paris 2024 and World Triathlon are confident that water quality will return to below limits,” they said. “As observed in July, with summer conditions (more sunshine, higher temperatures, prolonged absence of rain) water quality in the Seine has improved significantly.”

Swimming in the Seine has been banned for over a century. Since 2015, organisers have invested around £1bn to prepare the Seine for the Olympics and to ensure Parisians have a cleaner river after the Games.

The plan has included constructing a giant underground water storage basin in central Paris, renovating sewer infrastructure and upgrading wastewater treatment plants.