Uncooked curry leaves in a chutney left more than 400 people who ate at a street food festival with diarrhoea and vomiting or salmonella poisoning, health officials have found.
The leaves were contaminated with several different bacteria, experts found, which led to 29 confirmed cases of salmonella at the Street Spice Festival in Newcastle in February and March.
An investigation by Public Health England and Newcastle City Council found 25 of the 29 cases had developed a strain of salmonella never found in people or food in this country before.
According to an official report, further laboratory analysis suggested other organisms may also have caused illness including E.coli and Shigella.
Some of the 413 affected were found to have more than one of these infections at the same time.
No-one will face prosecution because there was seen to be a lack of clear advice about the dangers of using raw curry leaves in recipes, and in general hygiene levels at the three-day event were good.
Dr Kirsty Foster, chair of the outbreak control team and consultant in health protection with PHE said: "This was one of the largest outbreaks of gastro-intestinal illness associated with herbs or spices in the country.
"In addition, it was the first time one of the strains of salmonella was detected in the UK.
"However, herbs and spices are known to be potential sources of salmonella and other organisms, and have been reported in scientific literature as the source of infection in a number of outbreaks across the country.
"But it is unclear whether there is widespread understanding among food handlers and the public about the potential for infection when using these products raw.
"That is why we have reported our findings to the Food Standards Agency, recommending that advice is developed for the food industry and the public about the use of raw curry leaves.
"While this is being developed, our advice to the public is to cook curry leaves thoroughly if they are to be used in recipes and to be aware of the risk of infection if using them raw."
Stephen Savage, Newcastle City Council's director of regulation and public protection said: "Our environmental health officers have worked very closely with experts in health protection since the outbreak of this illness.
"We have carried out extensive investigations and tests and have identified the source of this outbreak as being imported contaminated curry leaves used raw in a chutney.
"Having carefully considered the facts, we have decided against formal action in this case.
"This decision, based on our enforcement policy, takes into account the lack of clear, official advice about the use of curry leaves and the overall good standards of food hygiene at the festival.
"We have recommended to the Food Standards Agency that they should develop information to be circulated nationally about the preparation of curry leaves and other herbs.
"We will ensure that this advice is given to the local restaurant trade.
"We will carry out further work with the Food Standards Agency and Public Health England to examine more closely the extent of contamination in curry leaves and other herbs imported from abroad.
"We are also recommending to the Food Standards Agency that awareness is raised with importers, exporters and port health authorities about the labelling, and in particular the instructions for use of curry leaves and other herbs."
Dr Foster thanked the hundreds of customers who contacted the inquiry team after the outbreak.
"I would also like to thank the festival organisers who co-operated fully with the investigation at all times, under very difficult circumstances," she added.