Officials made the decision in the early hours of the morning after tests revealed that recent rainfall had led to sewage contaminating the river.

The condition of water has been a subject of frequent controversy in the run-up to the Olympics, with warnings that pollution could harm the athletes.

Let's speak to Simon Ward, he's a high-performance coach who helps athletes train for events such as triathlons or Ironmans.

Simon, how common is it for events like this to be affected by water issues such as this?

In practice, how would it affect the swimmers if they'd gone ahead today?

We saw a few Parisian politicians swim in the river beforehand to show that it would be OK.

Could the situation have changed so quickly?

I think the Parisians have been a little unlucky, they've done a lot to make the water better and the weather's impacted that really, and again if you're outside for sporting events the weather can play a part.