A new COVID variant spreading through southern African countries has prompted calls for the government to act quickly and add them to the red list to prevent its arrival in the UK.
The variant - called B.1.1.529 - has a "very unusual constellation" of mutations, which are concerning because they could help it evade the body's immune response and make it more transmissible, South African scientists have said.
Dr Tom Peacock, a virologist at Imperial College London, said the variant could be “of real concern” because of its "horrific spike profile".
South African authorities said the variant had already spread through Johannesberg and could have made it to all corners of the country.
South Africa has confirmed around 100 cases of the variant but it has also been found in Botswana (where it was first detected) and Hong Kong.
Watch: COVID-19: 'Really awful' new coronavirus variant B.1.1.529 identified in Botswana
Professor Christina Pagel from University College London called on South Africa to be red-listed due to the sharp increase in detection of the variant among positive tests in recent days.
She said the data suggested the new variant may have significant advantages over Delta - the current dominant strain in the UK and many other areas of the world.
She added the mutations might mean it can significantly get around the vaccines.
Dr Pagel said: "It's hopefully not here yet. We don't have definitive evidence on transmission advantage or immune escape but we have plenty of cause to suspect both.
"Let's be super protective of our vaccine programme and take precautionary action.
Ewan Birney from the European Molecular Biology Laboratory said the new variant "does definitely look like a code red" and called for a South Africa travel ban.
Doctor Anthony Costello made a similar call saying the variant was "causing great concern."
The World Health Organization said in a statement it was monitoring the development of the variant.
They said: "So far under 100 sequences have been reported. Early analysis shows that this variant has a large number of mutations that require and will undergo further study."
The WHO said they would convening a meeting on Friday "to better understand the timeline for studies that are underway and to determine if this variant should be designated as a variant of interest or variant of concern."
The potential spread of a new risky variant will draw parallels to the spread of the Delta variant that originated in India earlier this year.
At the time Boris Johnson was harshly criticised for not putting India on the red list soon enough despite urgent warnings from experts.
Downing Street said a coronavirus variant would be kept under “close investigation”.
A Number 10 spokesman said: “We continue to monitor new variants as they emerge with our partners around the world.
“We have one of the largest genomic sequencing programmes here in the UK that allows us to spot and track variants as they emerge and, as we have done throughout the pandemic, we will continue to keep an eye and keep this particular variant under investigation.”
Asked whether travel restrictions would be needed before Christmas as a result of the variant, the spokesman said: “We will continue to keep the latest situation, the latest scientific evidence and data, under review, as we have done throughout the pandemic.
“We have said before if we believe we need to take action we will, but we will continue to monitor this variant and other variants in the same way that we have done throughout the pandemic.”
The UK effectively ended the traffic light system for travel when it took all countries off the red list at the end of October.
South Africa was the first country to detect the Beta variant last year, which is only one of four the WHO's 'variants of concern.'