A newly identified coronavirus variant that has spread in South Africa is the most concerning that British health officials have yet seen as it has double the number of mutations of the Delta variant, including some associated with evading immune response.

The UK announced it was temporarily banning flights from South Africa and five other countries from 1200 GMT on Friday and returning British travellers from those destinations would have to quarantine.

The UK Health Security Agency said that the variant - called B.1.1.529 - has a spike protein that was dramatically different to the one in the original coronavirus that COVID-19 vaccines are based on.

It has mutations that are likely to evade the immune response generated both by prior infection and vaccination, and also mutations associated with increased infectivity.

The variant has also been found in Botswana and Hong Kong.

Israel has also announced it is barring its citizens from travelling to southern Africa and banning the entry of foreigners from the region over the new 'super variant'. The other countries the UK has blocked flights from are Namibia, Lesotho, Botswana, Eswatini and Zimbabwe.

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Lab studies are needed to assess the likelihood of the mutations resulting in greatly reduced vaccine efficacy, scientists said. It's believed the rapidly spreading variant could potentially reduce the effectiveness of vaccines to as little as 30 percent.

Officials have advised the UK government on the need to act swiftly and pre-emptively in case the concerns over the impact of variant are borne out, even though it could take weeks to generate all the information needed about its characteristics.

"The early indication we have of this variant is it may be more transmissible than the Delta variant and the vaccines that we currently have may be less effective against it," said UK Health Secretary Sajid Javid.

"We've always been clear that we will take action to protect the progress that we have made."

South African scientists said they had detected the new COVID-19 variant in small numbers and were working to understand its potential implications.

The variant has a "very unusual constellation" of mutations, which are concerning because they could help it evade the body's immune response and make it more transmissible, the scientists told reporters at a news conference.

The UK Health Security Agency said that no cases of the variant had been detected in Britain and they were in contact with South African colleagues over their data.