Around 200 people who attended a spicy food street festival have reported suffering from diarrhoea and vomiting.
Early tests have shown four people suffering from salmonella following the Street Spice festival in Newcastle's Times Square, which finished on Saturday.
Environmental health Officers and the Health Protection Agency investigated after scores of people fell ill.
A council spokesman said: "In the first batch of tests, four people have proved positive for salmonella. Further results will be available in due course.
"It is estimated that of the 12,000 people who attended the event, up to 200 have reported sickness."
Stephen Savage, director of regulatory services and public protection at Newcastle City Council, said: "The event organisers are cooperating fully and we are continuing to investigate the source of the outbreak.
"Please can anyone with symptoms contact Regulatory Services and Public Protection at the City Council on 0191 278 7878."
Dr Kirsty Foster, of the Health Protection Agency and chair of the outbreak control team, said: "Initial investigations have not yet identified a definite source of infection, however, we are working closely with the organisers of the event to determine the source of infection.
"Anyone who is concerned about symptoms suggesting salmonella infection should contact their GP or out-of-hours service in the first instance.
"Those affected should not return to their place of work until their symptoms have ceased for 48 hours."
Most people recover from salmonella without treatment, but occasionally the illness can become more serious and require hospital admission.