Travellers arriving from several southern African countries will have to quarantine amid warnings over a new Covid variant.
Health Secretary Sajid Javid said from 12:00 GMT on Friday six countries would be added to the red list, with flights being temporarily banned.
One expert described the variant, known as B.1.1.529, as “the worst one we’ve seen so far”, and it is thought to have the potential to evade immunity.
No cases have been confirmed in the UK.
Only 59 confirmed cases have been identified in South Africa, Hong Kong and Botswana so far.
All flights from South Africa, Namibia, Zimbabwe, Botswana, Lesotho and Eswatini are being suspended, Mr Javid said.
He said that scientists were “deeply concerned” about the new variant but more needed to be learnt about it.
But he said that what we do know is that there is a significant number of mutations, “perhaps double the number of mutations that we have seen in the Delta variant”.
He added: “And that would suggest that it may well be more transmissible and the current vaccines that we have may well be less effective.”
He said adding the six countries to the red list was about “being cautious and taking action and trying to protect. as best we can, our borders”.
Experts from the World Health Organization (WHO) are meeting with South African officials on Friday to assess the evolving situation in the country.
The new variant is heavily mutated and the confirmed cases are mostly concentrated in one province in South Africa.
Scientists say the variant has 50 mutations overall and more than 30 on the spike protein, which is the target of most vaccines and the key the virus uses to unlock the doorway into our body’s cells.
One scientist told me this was the worst variant they’d seen – look at it on paper and it’s not hard to see why.
It is the most heavily mutated variant so far and is now radically different to the form that emerged in Wuhan, China.
That means vaccines, which were designed using the original, may not be as effective.
And some of its mutations are known to increase the ability of coronaviruses to spread.
But there have been many variants that looked bad on paper before, but haven’t taken off.
There are early signs this virus is spreading in South Africa and may already be in every province in the country.
But the big questions – how much does it evade vaccines, is it more severe, does it spread faster that Delta – are unanswerable for now.
All remaining countries were removed from the UK’s red travel list in October.
Each individual nation sets its own policy but all parts of the UK have generally followed the same rules for past announcements.
Currently people travelling to the UK who are fully vaccinated do not need to take a Covid test before setting off but do need to take a lateral flow test two days after arrival and fill out a passenger locator form.
Those who are not fully vaccinated and are over 18 must self-isolate at home for 10 days and take a Covid test in the three days before travelling as well as on day two and day eight of their return.