Health officials are said to be scrambling to trace people who have recently arrived from southern Africa amid fears over a new mutant strain of Covid.
Six countries have been added to the UK's red list after the discovery of the B.1.1.529 variant, which has the potential to evade immunity built up by vaccination or prior infection.
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.
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.
Do you have a story? Email firstname.lastname@example.org.
Health officials are said to be scrambling to trace arrivals
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."
Announcing the travel ban on Thursday, health secretary Sajid Javid said the new strain "may be more transmissible" than the Delta strain and "the vaccines that we currently have may be less effective".
He said: "Our scientists are deeply concerned about this variant. I'm concerned, of course, that's one of the reasons we have taken this action today."
Asked what the situation would mean for the UK over the coming weeks, with Christmas approaching, he continued: "We've got plans in place, as people know, for the spread of this infection here in the UK and we have contingency plans – the so-called Plan B.
"But today's announcement, this is about a new variant from South Africa – it's been detected in South Africa and Botswana – and this is about being cautious and taking action and trying to protect, as best we can, our borders."
He went on to say: "Now to be clear, we have not detected any of this new variant in the UK at this point in time. But we've always been clear that we will take action to protect the progress that we have made."
Mr Javid said as well as the suspension of flights, anyone that arrives from the six countries from 4am on Sunday must quarantine in hotels.
The red list had been scrapped earlier this month when all countries were taken off it.
Sign up for our daily coronavirus briefing newsletter to keep up to date with all the essential information and changes at www.mirror.co.uk/email .
Professor Adam Finn, a member of the Joint Committee on Vaccination and Immunisation, said more needs to be learned about a new variant of the virus.
Speaking on Friday, he told ITV's Good Morning Britain that a jump in cases in South Africa could be linked to the variant's enhanced transmissibility.
He said: "We now need to wait and see just what kind of threat this new variant may pose. If we're lucky, it won't be a serious one, but it could be very serious."
Travelers arrive at Heathrow Airport
He said he could not predict if this would affect Christmas.
He continued: "On the one hand, I don't want to induce unnecessary anxiety in people, but on the other hand, I think we all need to be ready for the possibility of a change in the restrictions.:
Prof Finn said sequencing is being carried out around the UK to determine if any cases have already been imported.
"There are a number of things going on now to understand this, to look for it, to trace it, to hopefully stamp it out if it is already here," he said.
Two Welsh rugby teams are looking to repatriate their staff following the travel ban, with Cardiff Rugby and Scarlets Rugby both in South Africa to play United Rugby Championship matches.
Simon Muderack, the executive chairman of Scarlets Rugby, tweeted on Thursday night: "Many will have seen the news about the new Covid variant that affects our people in South Africa.
"We are focused on the welfare of our staff in South Africa and the concern of their families at home. We are exploring all options to repatriate our people as soon as possible."
Cardiff Rugby released a similar statement, and promised to offer regular updates with any developments.