A travel ban has been introduced for passengers arriving from South Africa to England.
It comes after two cases of another new strain of Covid-19 linked to the country were identified in the UK.
From 9am on Christmas Eve (Thursday), visitors arriving in England who have been in or transited through South Africa in the previous 10 days will not be permitted entry.
Direct flights will be banned, the Department for Transport said.
The ban excludes cargo and freight without passengers.
It also does not include British and Irish nationals, visa holders and permanent residents, who will be able to enter but are required to self-isolate for 10 days along with their household.
On Wednesday, Health Secretary Matt Hancock said the variant is 'highly concerning' because it is 'yet more transmissible'.
It appears to have mutated further than the previous new variant that has been discovered in the UK recently.
During the Downing Street press conference, Dr Susan Hopkins, of Public Health England, said: "The new variant that we have identified in the UK is very different to the variant in South Africa, it's got different mutations.
"Both of them look like they are more transmissible.
"We have more evidence on the transmission for the UK variant because we've been studying that in great detail with academic partners.
"We are still learning about the South African variant.
"We are pretty confident that the system we have in place will help control the spread.
"Clearly this needs to be continually monitored and we have a genomic system in place to do that.
"And we are looking at constantly enhancing the detection of cases in travellers."
