About 2,000 people participated in the events last weekend, which included a swim off Sunderland’s blue flag Roker beach. The UK Health Security Agency (UKHSA) said it would be testing samples from those who were ill to establish the cause of the illness and any common pathogens.

An Environment Agency sampling at Roker beach on Wednesday 26 July, three days before the event, showed 3,900 E Coli colonies per 100ml, more than 39 times higher than typical readings the previous month. E coli is a bacterial infection that can cause stomach pain and bloody diarrhoea.

But British Triathlon, the governing body for triathlons in Great Britain, said 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.

The event was on a stretch of coastline that has been at the centre of a long-running battle over sewage discharges between campaigners and the government concerning regulatory failures.