The number of people who fell ill following a graduation ceremony event held by the Royal College of Art in London has risen to more than 250, however Public Health England said it had received no reports of long-term illness.
The government body said attendees had experienced a gastrointestinal illness, with norovirus confirmed in some cases, but the cause remains unknown.
Symptoms experienced by those who have fallen ill have been "short and self-limiting and an investigation is ongoing.
Dr Deborah Turbitt, deputy director of PHE London, said: "People feeling unwell should isolate themselves as far as possible, and wash their hands when handling food or using the toilet in order to prevent the spread of infection. Those with diarrhoea or vomiting should take care to drink plenty of fluids to stay hydrated."
The NHS advises that norovirus can be spread by close contact with someone with the illness, touching surfaces or objects that have been touched by someone with norovirus or eating food that has been prepared or handled by someone with the sickness bug.
The exact location of where the event took place has not been revealed, although it has been confirmed that CH&Co was the caterer at the event.
A spokesperson for CH&Co said: "We are aware that Public Health England and Westminster City Council are carrying out investigations into an incident at the RCA and we are actively assisting them in their enquiries in order for them to bring the investigations to a swift conclusion. Whilst these investigations are taking place, it would be inappropriate to provide further information at this stage."