Many food stalls at music festivals, carnivals and sports events contain bacteria indicating the presence of human or animal faeces.
A study by the Health Protection Agency looked at bacteria found in food, chopping boards, cleaning cloths and security wristbands in outdoor food vendors.
They found many were contaminated with a range of bacteria, including E.coli and Salmonella.
The HPA collected 1,662 samples from 153 events, including 50 concerts or music festivals, 20 sports events and 39 carnivals, fetes and fares, as well as 44 unclassified events.
Twenty seven percent of the water samples contained unacceptable levels of coliform bacteria, which can be a sign of faecal contamination.
E.Coli was found in eight percent of water samples tested, as well as on some cleaning cloths.
Eight percent of food samples were found to be of an unsatisfactory quality. One percent contained potentially hazardous levels of bacteria, including the presence of Salmonella, which normally occurs when food is cooked and then kept warm for hours before serving.
Samples from chopping boards were found to have the most unsatisfactory levels of contamination, with 60 percent of samples not meeting the required standard of hygiene.
A fifth of wristbands worn by vendors were contaminated with Enterobacteriacae, E.coli or Staphylococcus, which lives on the skin and can cause food poisoning.
Caroline Willis, a specialist microbiologist at the HPA's Food, Water and Environment laboratory in Porton Down, said: "Gastrointestinal illnesses are some of the most common problems encountered by people attending festivals and large-scale outdoor events.
"Various studies have looked at the microbiological standards of food and environments in such locations and although this study showed some improvement in standards of cleanliness there is clearly a lot of room for improvement.
"There are various reasons why hygiene is lower at such events including the volume of customers, use of temporary staff, working in cramped conditions, lack of storage space and difficulties with on-site cleaning.
"These all combine to lead to greater cross-contamination risks which can be increased if levels of personal hygiene are not good.
"Local Environmental Health Departments have done much to improve standards at mobile and outdoor catering premises over recent years but staff need to ensure that both cooking standards and thorough hygiene are rigorously maintained to avoid the risk of people becoming unwell."