UK Health Secretary Sajid Javid said there were concerns a new coronavirus variant detected in South Africa “may be more transmissible” than the dominant Delta strain.
Announcing new restrictions on travel to Britain from six countries including South Africa, Javid suggested current Covid-19 vaccines "may be less effective" against the variant.
The coronavirus evolves as it spreads and many new variants, including those with worrying mutations, often just die out.
Scientists monitor for possible changes that could be more transmissible or deadly, but sorting out whether new variants will have a public health impact can take time.
Currently identified as B.1.1.529, the new variant has also been found in Botswana and Hong Kong in travellers from South Africa.
The World Health Organization's technical working group is to meet on Saturday to assess the new variant and may decide whether or not to give it a name from the Greek alphabet.