The individual, who is no longer in the country, tested positive after travelling here and “is linked to travel to southern Africa“.
The UK Health Security Agency (UKHSA) is now carrying out targeted testing in places they visited when they were likely to be infectious, including, it is thought, Westminster.
Dr Jenny Harries, UKHSA chief executive, said: “Our advanced sequencing capabilities enable us to find variants and take rapid action to limit onward spread.
“It is very likely that we will find more cases over the coming days as we are seeing in other countries globally and as we increase case detection through focused contact tracing.”
She said UK officials are continuing their efforts to understand the effect of the variant on transmissibility, severe disease, mortality, antibody response and vaccine efficacy.
“It's critical that anyone with COVID-19 symptoms isolates and gets a PCR test immediately,” she said.
“Vaccination is critical to help us bolster our defences against this new variant – please get your first, second or booster jab without delay.”
She urged people to wear a mask in crowded places, including public transport and shops, “to ensure we all help break the chains of transmission and slow the spread of this new variant”.
Mandatory mask-wearing will return to shops and public transport in England on Tuesday.
This is the third Omicron case to be detected in the UK after the health secretary announced two others on Saturday.
Sajid Javid said there would also be targeted testing in the areas where the first two cases were found – in Brentwood, Essex, and in Nottingham.
Mr Javid said the two individuals were self-isolating along with their households while further tests were carried out.
The two cases were “linked”, the health secretary said, and that connection was traced to South Africa.
The variant has prompted the UK to place ten African countries on the red list – South Africa, Namibia, Lesotho, Botswana, Eswatini, Zimbabwe, Angola, Mozambique, Malawi and Zambia.Internet Explorer Channel Network