The customers visited the pub on Friday (11 September) and ordered 18 oysters alongside a wider meal.
A spokesperson for Brighton & Hove City Council, which is leading the investigation, said: “We can confirm we are investigating complaints from a party who were at the establishment on Friday.
“We have visited the premises and are satisfied with the standards of hygiene there.”
An internal investigation by the Urchin said the only realistic causes of the incident could have been a localised bug in a small number of oysters that went undetected, a combination of oysters and spirits - which can pickle oysters in the stomach, or that one of the party had unknowingly picked up a bug during the day and inadvertently spread it to the other diners.
Nick Jerrim, general manager of the Urchin, said: “Both the environmental health officer and the restaurant chef and manager believe that they operated within best practice guidelines at all times and could not have done any more to prevent this incident.
“The management are very sorry that their guests were ill after attending the restaurant and have apologised to all of those impacted by this incident, but they do not believe that there was anything further they could have done to prevent this.”
The oysters are reported to have come in a batch of 5,000, of which 400 were distributed by Fish Galore, a supplier in Hove, to food businesses including restaurants and pubs across the city - all of them are believed to have been consumed and no further complaints have been received.
Oysters are traditionally eaten raw and considered a delicacy and aphrodisiac.
However, the Food Standards Agency (FSA) has previously issued warnings over the possible risk of getting “winter vomiting bug” norovirus from eating them raw.
A spokesperson for the FSA said: "Though oysters are traditionally eaten raw, people should be aware of the risks involved in eating them in this way.
"The agency advises that older people, pregnant women, very young children and people who are unwell should avoid eating raw or lightly cooked shellfish to reduce their risk of getting food poisoning."
In 2011, research conducted by the FSA reported that more than three quarters of oysters from UK growing beds contained the bug, which can cause vomiting and diarrhoea.