At least 57 participants fell sick and suffered diarrhoea after competing in sea swimming events at the World Triathlon Championship Series in Sunderland.
After a swim off Sunderland's Roker beach last weekend, those who fell ill complained that they were suffering from sickness and diarrhoea.
Around 2000 people participated in the race which also included a family event.
The Guardian report that an Environment Agency sampling at Roker beach three days before the event, showed a reading for 3,900 E Coli colonies more than 39 times higher than the previous month.
Australian triathlete Jacob Birtwhistle posted on Instagram earlier this week that he felt unwell after the race.
"Have been feeling pretty rubbish since the race, but I guess that's what happens when you swim in s***," adding that he believed the swim should have been cancelled.
Another wrote: "That now explains why I spent Monday night with my head in the toilet after racing Sunday morning!"
The triathlon event in Sunderland served as the British leg of the 2023 World Triathlon Championship Series and coincided with the qualification period for the Paris 2024 Olympic Games.
The national governing body, British Triathlon, stated on Thursday its commitment to collaborating with Sunderland City Council and the health authorities to establish the cause of the illnesses.
