British Prime Minister Boris Johnson unveiled on Saturday new measures to try to stop the spread of a new coronavirus variant called Omicron, saying all arrivals would have to take a test and it was time to step up booster jabs.
"We will require anyone who enters the UK to take a PCR test by the end of the second day after their arrival and to self-isolate until they have a negative result," Johnson told a news conference.
He said those who had come into contact with people testing positive for a suspected case of Omicron would have to self-isolate for 10 days and that the government would tighten up the rules on wearing face coverings.
England's Chief Medical Officer Chris Whitty told the same news conference there was a reasonable chance that the newly identified variant could be less easy to tackle with vaccines.
"There is a reasonable chance that at least there will be some degree of vaccine escape with this variant," he said.
Health minister Sajid Javid confirmed earlier on Saturday the first two cases of the new variant in Britain, saying they were linked to each other and connected to travel to southern Africa.
The two individuals and all members of their households were being re-tested and told to self-isolate while further testing and contact tracing was done.
England will also add Malawi, Mozambique, Zambia and Angola to its travel "red list" from 0400 GMT on Sunday, meaning British and Irish residents who arrive in the country must quarantine in a government-approved hotel for 10 days. Non-residents will be refused entry.
That list already contained Botswana, Eswatini, Lesotho, Namibia, South Africa and Zimbabwe.