A third case of the Omicron Covid variant was detected in the UK on Sunday, with the infected individual said to have visited central London while they were harbouring the virus.
Health officials said they had been in Westminster shortly before leaving the country and the infection was “linked to travel to southern Africa”.
The UK Health Security Agency (UKHSA) announced it would carry out targeted testing in the areas visited by the individual. It brought the total number of infections of the variant to three after two were confirmed in Essex and Nottinghamshire on Saturday.
Dr Jenny Harries, UKHSA chief executive, said: “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.
"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."
Boris Johnson has announced fresh restrictions to curb the spread of the variant, which scientists fear could be more transmissible and potentially resistant to vaccines.
Passengers arriving in the UK have been told that from Tuesday morning they will have to take a PCR test for Covid-19, with the expectation they will have to self-isolate until they test negative. Face masks will also be reintroduced in shops, on public transport and in communal areas in secondary schools.
All contacts with a suspected case of Omicron will also have to isolate for 10 days, regardless of their vaccination status.
Health secretary Sajid Javid on Sunday said he hoped the extra measures would be “temporary” and that they would encourage Britons to take the virus “more seriously”.
Senior scientists have said it is the worst strain they have seen so far during the pandemic and a string of countries have banned flights from southern Africa to prevent its spread.
Despite the reintroduction of rules, Mr Javid told families they should plan for a great Christmas “as normal” and insisted it was “nowhere near” time to reintroduce social distancing rules and work-from-home guidance.
As officials tried to trace potential contacts of the Omicron case in Essex, the local authority urged visitors to a KFC in Brentwood on November 19 to get drive-through tests, raising questions about how long ago the variant may have arrived in the UK.
Meanwhile, the doctor who discovered the Omicron variant said the UK is panicking unnecessarily, while South Africa’s health minister said travel restrictions on his country are “uncalled for”.
Dr Angelique Coetzee, chair of the South African Medical Association, told the programme she first encountered the variant in a man in his early 30s who presented with tiredness and a mild headache, but none of the usual coronavirus symptoms.
She said: “What we are seeing clinically in South Africa, and remember I’m at the epicentre – that’s where I’m practising – it’s extremely mild. For us, that’s mild cases.”