A third case of the Omicron Covid-19 variant has been confirmed in the UK - in a person who visited London.

The UK Health Security Agency said  that the individual, who is no longer in the country, tested positive after travelling to Britain and "is linked to travel to southern Africa".

The UK Health Security Agency (UKHSA) is now carrying out targeted testing where the individual visited when they were likely to be infectious.

They are said to have been in Westminster in central London.

The UK Health Security Agency added that it was "very likely that we will find more cases over the coming days... as we increase case detection through focused contact tracing".

The first UK cases were confirmed on Saturday in Brentwood, Essex and Nottingham.

Officials said the cases were linked and connected to travel in southern Africa.

Early evidence suggests the variant - first reported to the World Health Organization from South Africa on Wednesday - has a higher re-infection risk.

On Saturday, the government announced that face masks will be compulsory in shops and on public transport in England, and UK arrivals are expected to have to take PCR tests from Tuesday in response to the new variant.

Health Secretary Sajid Javid said they had acted "swiftly" and "in a proportionate way".

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 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."

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