GPs across Norfolk and Suffolk have been warned to look out for any symptoms of a stomach upset, although so far no cases have been linked back to the site at Henham Park, near Southwold.

The Health Protection Agency (HPA) confirmed on Wednesday 20th July that water supplies at the three-day festival over the weekend, tested positive for E.coli.

Organisers have said it affected non-drinking water and that precautions had been taken. How the E.coli got into the water supply is still being investigated.

Problems were discovered following routine testing and food traders were immediately advised to boil and/or chlorinate the water. Warning notices were put up around the site, encouraging people to dispose of any stocked-up water and to report any symptoms of diarrhoea, stomach cramps or vomiting to on-site medics.

A spokesman for the event organiser Festival Republic said: We were not prepared to take any risks at all when it came to public health, hence the supply of alternative drinking water as a precautionary measure.

The number of people that saw the medics with diarrhoea or vomiting at Latitude Festival 2011 was lower than the numbers expected at a comparable festival with no water contamination issues and was 33 per cent lower than the numbers that presented in 2010.