The new super mutant Covid variant could already be in the UK, health chiefs have warned.
UK officials are said to be scrambling to trace people who have recently arrived from southern Africa after the strain has been called the 'worst ever seen'.
New travel restrictions will be put in place with six counties being added to the red list after concerns over the potential 'vaccine evading' variant.
Hundreds of people who landed from these countries in the last 10 days are now being urged to take a PCR test - although no cases are confirmed so far.
The B.1.1.529 variant has first been found in cases in Botswana, South Africa and Hong Kong.
Despite only being tracked for the past three days, the virus has been identified as having 30 different mutations already.
By comparison, that is twice as many as the Delta variant, which has been the most prominent variant in the UK over the past few months.
The bulk of new cases have been found in South Africa, but arrivals from Namibia, Lesotho, Botswana, Eswatini and Zimbabwe will also be temporarily banned from 12pm on Friday.
The variant has also not yet been labelled "of concern" in the UK - but one senior UK Health Security Agency (UKHSA) expert said: " This is the worst variant we have seen so far."
Asked if it could already be in Britain, she told the BBC : “It’s always possible.”
But she added: “We have no cases identified whatsoever yet.”
From 4am on Sunday all travellers from these destinations will have to spend 10 days in quarantine hotels.
British health officials are scrambling to trace anyone who has returned from these countries in recent days to see if they are carrying the new strain, which is expected to be named 'Nu'.
Scientists as they do not have enough evidence on its levels of transmissibility, however, some have said they are concerned.
It is known as a “variant under monitoring”, meaning scientists believe it may pose a future risk, but its impact is unclear.
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