Sajid Javid has said the new Covid variant “may pose substantial risk to public health”.
Speaking in the Commons on Friday, the health secretary said there were “very live” discussions in government over adding further countries to the red list.
Javid said there were currently no UK cases of the variant, known as B.1.1.529, found in South Africa, Botswana, Hong Kong and Israel.
But he said it was “of huge international concern” and early indications showed the current vaccines “may be less effective against it”.
The UK has already added South Africa, Botswana, Lesotho, Eswatini, Zimbabwe and Namibia to its travel red list.
Passengers arriving into the UK from these countries from 4am on Sunday will be required to book and pay for a government-approved hotel quarantine for 10 days.
Transport Secretary Grant Shapps said ministers were taking a “safety-first approach”.
Dr Susan Hopkins, chief medical adviser of the UK Health Security Agency (UKHSA) said the variant was the “most worrying” seen by scientists but much was as yet unknown.
South African scientists sounded the alarm over the variant, which they fear is behind a spike in cases in some regions, including the Gauteng province, which includes the cities of Pretoria and Johannesburg.
Experts from the World Health Organisation are meeting with South African officials on Friday to assess the evolving situation in the country.