Two cases of the Covid Omicron variant have been identified in the UK with the people involved linked to each other and to travel to southern Africa, the Government said.

Tweeting, heath secretary Sajid Javid said: "As a precaution we are rolling out additional targeted testing in the affected areas and sequencing all positive cases. This is a fast-moving situation and we are taking decisive steps to protect public health.

"We are also adding Malawi, Mozambique, Zambia and Angola to the travel red list - effective from 4am Sunday. If you have returned from there in the last 10 days you must isolate and get PCR tests. And if you are eligible for your booster jab - now is the time to get it."

The cases have been found in Nottingham and Brentwood.

A scientist behind the Oxford jab has expressed optimism that existing vaccines will be effective against the new Omicron variant and said it is “extremely unlikely” it will cause a “reboot” of the pandemic in Britain.

Professor Sir Andrew Pollard, who helped create the Oxford/AstraZeneca vaccine, said experts would need to wait “several weeks” for confirmation but said existing jabs could still be effective at presenting serious illness.

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His remarks came as countries rushed to close their borders to nations in southern Africa to slow the spread of the strain designated a “variant of concern” by the World Health Organisation over fears it presents an increased risk of reinfection.

The UK Government has banned flights arriving from South Africa and five neighbouring nations, but ministers were considering adding more countries to the red list, as Belgium became the first EU country to report a case of Omicron.

The Netherlands was also of concern, with Dutch authorities saying 61 people tested positive for Covid-19 arriving on two flights from South Africa on Friday. Further tests are under way to determine if any of them had the Omicron variant.

Numerous pharmaceutical firms have said they are working to adapt their vaccines in light of the emergence of Omicron after the WHO warned that preliminary evidence suggests the variant has an increased risk of reinfection and may spread more rapidly than other strains.