An investigation has been launched after two confirmed cases of Omicron were linked to a private party in Somerset.
The UK Health Security Agency South West said there were further suspected cases linked to the event.
Its deputy director Dominic Mellon said it was following up with any contacts.
He spoke after Health Secretary Sajid Javid confirmed the Omicron variant had been found in “multiple regions of England”.
It has not been revealed in which part of Somerset the party took place.
However, epidemiologist Tim Spector claimed it happened at a 60th birthday celebrations.
He tweeted that all the guests were vaccinated and some had received booster jabs at the “super-spreader event”.
Between 14 and 18 people later tested positive for the Covid variant – first recorded in South Africa – “but symptoms all mild luckily”, he claimed.
Mr Mellon said it was incredibly important to “emphasise the importance of self-isolation and testing”.
“It is critically important that we do everything we can to help stop the variant’s spread while we learn more about how infectious it is, whether it could cause more severe illness, and how effective our vaccines are against this variant.”
He advised people to take lateral flow tests before attending social events and meet “outdoors if you can, or if indoors, open a window to let in fresh air.”
“All of these individual actions can make a difference to help slow the spread of the infection and help keep each other safe.
“New variants can spread very quickly and we need everyone to remain cautious and take steps to keep Covid-safe, particularly at a time when our overall coronavirus rates remain high.
“Get vaccinated on schedule – first dose, second dose, booster. Wear a face covering on public transport, in shops and crowded places.
“If you have symptoms, isolate immediately and arrange a PCR test,” he said.
