The UK Health Security Agency (UKHSA) is now attempting to track the virus by carrying out urgent testing in areas they visited when they were likely to have been infectious.
This includes Westminster in central London.
The person tested positive after travelling to Britain from southern Africa but they have since left the UK.
UKHSA chief executive Dr Jenny Harries warned that further cases are “very likely”.
She 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 focussed contact tracing.
“We are continuing our efforts to understand the effect of this 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.
“Vaccination is critical to help us bolster our defences against this new variant – please get your first, second or booster jab without delay.
“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.”
The first two Omicron infections were reported in Chelmsford, Essex, and Nottingham.
UPDATES TO FOLLOW…