Twelve confirmed and 57 probable cases of E.coli have been linked to a restaurant in north Belfast, health authorities say.
The diners were struck down after eating at Flicks in Cityside Mall, York Street, in the last three weeks.
Management at the restaurant have voluntarily closed the premises and are co-operating with an investigation by the Public Health Agency (PHA) and environmental health officers from Belfast City Council.
The PHA has advised that anyone who ate at Flicks since September 24 and has symptoms of diarrhoea and/or abdominal pain should contact their GP urgently for medical advice.
The agency has revealed that four cases of the same E.coli O157 strain were linked to the restaurant in August. But it said subsequent tests in the restaurant at that time were negative.
Further environmental health inspections were conducted after those incidents, all of which were satisfactory.
The PHA said enhanced surveillance to monitor for cases was also put in place and the current cases came to light through that system.
The agency said it was examining whether the current outbreak was linked to the August instances but it was too early to confirm either way.
Dr Brian Smyth, public health consultant with the PHA, said: "The Escherichia coli bacteria is commonly found in the intestines of humans and animals. There are many different types of E.coli, and while some live in the intestine quite harmlessly, others may cause a variety of diseases. The bacterium is found in faeces and can survive in the environment.
"Symptoms caused by E.coli O157 can include diarrhoea, which can range from mild to profuse watery or bloody diarrhoea, tummy cramps, nausea or vomiting."