At least 57 triathletes fell ill with diarrhoea and vomiting after taking part in swimming competitions in Sunderland, health officials have said.
About 2,000 people took part in the UK leg of the World Triathlon Championship series in Roker last weekend, including a swim at the beach.
The UK Health Security Agency said it would send those with symptoms a questionnaire and ask them to send a sample for testing to determine the cause of the illness.
It said in a statement: "UKHSA's north east health protection team is working with British Triathlon and Sunderland City Council following reports of diarrhoea and vomiting in a number of participants who took part in the UK leg of the World Triathlon Championship Series in Sunderland.
"Organisers of the event have written to all participants to ensure they are aware and to advise anyone with symptoms to seek medical advice. The risk to the wider public is very low."
Environment Agency sampling at the beach in late July detected 39 times the amount of e-coli found in the water during typical readings.
E-coli is a bacterial infection that can cause stomach pain and bloody diarrhoea.
Australian triathlete Jake Birthwistle, who competed in the event, posted the Environment Agency's results on Instagram and said he had felt ill after the event.
He said: "Have been feeling pretty rubbish since the race, but I guess that's what you get when you swim in s**t. The swim should have been cancelled.
"At least I know now what got me and a bunch of other athletes who raced sick and ill. Well done and I hope you get better soon too."
Another said: "That now explains why I spent Monday night with my head in the toilet after racing Sunday morning!"
The stretch of coastline where the swim took place has been at the centre of a long-running dispute between campaigners and the Government over sewage discharges.
The Environment Agency said water off the beach was classed as excellent last year based on samples taken in the previous four summers.
British Triathlon, the governing body for UK triathlons, told The Observer the agency's sampling results were not published until after the weekend's events and were outside the body of the water where its competitions took place.
It said its own testing results passed the required standards for the event.
Northumbrian Water insisted there have been no sewage discharges at the beach since 2021.
An Environment Agency spokesperson said: "We routinely sample and monitor bathing waters, including at Roker, to provide information for bathers and support our ongoing work to improve their condition.
"The beaches at Roker and Seaburn were both classified as 'excellent' last year based on samples taken throughout the season from May to September over the last four years.
"Temporary dips in water quality can be caused by several factors, including heavy rain.
"We will continue to investigate pollution sources and drive improvements to ensure cleaner and healthier waters for people to enjoy."
A Northumbrian Water spokesperson said: "We have had no discharges from any of our assets that might negatively impact water quality at either Roker or the neighbouring Whitburn North bathing water since October 2021.
"Both bathing waters were designated as 'excellent' in the latest Defra classifications and sampling to date in the current season indicate this high quality is being maintained."
British Triathlon has been contacted for comment.
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