third case of the Omicron Covid variant has been detected in the UK.
The individual, who is no longer in the UK, visited central London while they were infectious.
Health officials said they had been in Westminster in central London and the infection was “linked to travel to southern Africa”.
The UK Health Security Agency (UKHSA) is now carrying out targeted testing in the areas visited by the individual.
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.”
The first two cases of the variant were detected in Nottinghamshire and Essex on Saturday.
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 and on public transport.
All contacts with a suspected case of Omicron will have to isolate for 10 days, regardless of their vaccination status.
Mr Javid said he hoped the extra measures will be “temporary”, adding he thinks people will “take this more seriously”.
The new B.1.1.529 variant, first detected in Botswana, has sparked alarm among scientists due to its high number of mutations.
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.