Many athletes have pulled out of the Rio Olympics due to fears over Zika Virus

Britain’s Olympic marathon swimmers, sailors and windsurfers have been told they could become seriously ill if they inadvertently take in contaminated water off Guanabara Bay.

Recent tests have shown that the quality of water in the Brazilian city is unhealthy and does not meet World Health Organisation safety standards.

Organisers have failed to clean up the sewage and household rubbish which flows untreated from a city of 12 million people into the contaminated Rio waters. The main worry is that competitors could end up with diarrhoea or violent vomiting.

Dr Daniel Becker, a Rio paediatrician, told the New York Times: “Foreign athletes will literally be swimming in human crap, and they risk getting sick from all those microorganisms. It’s sad but also worrisome.”

Tests taken last year recorded disease-causing viruses were extremely harmful.