A new, potentially more infectious variant of the novel coronavirus that causes COVID-19 has been found in Britain in cases linked to South Africa, British Health Secretary Matt Hancock said on Wednesday.

South Africa's health department said last week that a new genetic mutation of the virus had been discovered and might be responsible for a recent surge in infections there.

"Thanks to the impressive genomic capability of the South Africans, we've detected two cases of another new variant of coronavirus here in the UK," Hancock told a media briefing.

"Both are contacts of cases who have travelled from South Africa over the past few weeks."

Britain is already trying to curb the spread of a mutated strain of the virus which is up to 70% more transmissible, and further studies aerie being carried out on the new variant.

"This new variant is highly concerning, because it is yet more transmissible, and it appears to have mutated further than the new variant has been discovered in the UK," he said.

Close contacts of those with the new variant and all those who have been in South Africa in the last fortnight, or were in close contact with someone who had, must quarantine, he said.

Immediate restrictions were being imposed on travel from South Africa, he added.

Countries around the world have in recent days closed their borders to both Britain and South Africa following the identification of the new, fast-spreading variants of the coronavirus.

"So the new variant in the UK, which we've identified, is very different to the variant in South Africa, it's got different mutations," said Susan Hopkins from Public Health England.

"Both of them look like they're more transmissible. We have more evidence on the transmission for the UK variant because we've been studying that with great detail with academic partners. We're still learning about the South African variant."

She expressed confidence that the spread of the South Africa-linked variant would be controlled and said vaccines that have already been developed should be effective.

"We have no evidence at the moment that the vaccine will not work, so actually what that means in fact is that there's strong evidence that it will work, because the vaccine produces a strong immune response and it's broad and acts against lots of variation in the virus," she said.