Health chiefs are investigating why dozens of people suffered bouts of vomiting and diarrhoea after taking part in a mud run.
About 3,000 people took part in the event that involved open water swimming at the National Water Sports Centre in Nottingham.
Public Health England is trying to identify the cause of the illness, which affected a number of competitors earlier this month. Event organisers said water quality test results had been “normal”.
Some runners said they had suffered from sickness, diarrhoea, headaches, fever and aching limbs for up to five days after the event – blaming the quality of water at the venue.
As part of its investigation, Public Health England (PHE) has asked those affected to fill in a survey to help identify the cause of the illness and where it originated.
A PHE spokeswoman said in a statement that it was working with event organisers X-runner and the trust that runs the venue in an advisory capacity.
She said it was “unclear as to the cause of the illness and investigations are ongoing”.
Organisers X-runner described the event as “the ultimate mud run and obstacle course race”.
It involved 60 obstacles covering tough terrain and open water over a distance of 5k or 10k.
Competitors also had to swim, with buoyancy aids, in the rapids section of a white water course.
X-runner said all participants signed a disclaimer as part of the sign-up process and potential risks around water quality were outlined. It said the vast majority of people taking part have not been unwell.
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