The United Kingdom Health Security Agency (UKHSA) confirmed that some 57 participants suffered from diarrhoea and vomiting.

UKHSA guidance encouraged "anyone who participated and has or had symptoms after the event to contact the organisers."

"The team will undertake an investigation of cases as is routine when they are notified of gastroenteritis, this will involve a detailed questionnaire to establish more information on where illness may have been acquired," a UKHSA statement said.

Australia's 2018 Commonwealth Games gold medallist Jake Birtwhistle insisted the swimming should not have taken place. 

"Have been feeling pretty rubbish since that race but I guess that’s what you get when you swim in ****," Birtwhistle posted on social media.

He withdrew from the men's elite race after the swimming.

"British Triathlon and World Triathlon are aware of illness among some participants," a British Triathlon statement said. 

"We have communicated with participants regarding the situation and we will continue to work with Sunderland City Council and the UK Health Security Agency North East (UKHSA North East) in line with their routine processes to establish further information."

British Triathlon published certificates to show that water quality was approved by testing agencies before the event.

Paris 2024 is set to stage triathlon and Para triathlon test events next week on August 16 and 17, but a contingency plan has been revealed.

"In the unlikely event that water quality does not meet the requirement of World Triathlon and public health authorities, a contingency plan is in place which would see the race(s) shifted to a duathlon format," a World Triathlon statement said.

Last weekend, the World Aquatics Open Water World Cup event planned as a test event for Paris 2024 was abandoned after heavy storms affected water quality in the river Seine.  

"The water quality in the Seine has remained below acceptable standards for safeguarding swimmers' health," a French Swimming Federation statement said.

"The laboratory result confirmed that on Sunday morning, we were below the thresholds at all the survey points," the Paris Mayor's sports assistant Pierre Rabadan told Agence France Presse.  

Paris experienced its heaviest August rainfall since 1965.

"World Aquatics is disappointed that water quality in the Seine has resulted in the cancellation of the World Aquatics Open Water Swimming World Cup, but the health of our athletes must always be our top priority," World Aquatics President Husain Al Musallam said. 

In a joint statement, Paris 2024 organisers and city authorities insisted that "in recent weeks, water quality in the Seine has regularly reached the levels required for competitions to be held on the dedicated site, demonstrating the significant progress made."

Paris has pledged to undertake further measures to safeguard water quality.

World Aquatics warned, "Based on this weekend it is clear that further work is needed with Paris 2024 and local authorities to ensure robust contingency plans are in place for next year."

French authorities have already spent €1.4 billion (£1.2 billion/$1.5 billion) in cleaning the river, seen as a major Olympic and Paralympic legacy.

Olympic triathlon competitions are set to begin on July 30 2024 and continue on July 31 and August 5.

Olympic marathon swimming is scheduled on August 8 and August 9.