The coronavirus mutation first found in Britain has now spread to 50 other countries, while a similar South African-identified strain has now been found in 20 countries, according to the World Health Organization.

The agency also disclosed that a third new coronavirus “variant of concern” found in Japan may impact upon immune response and needs further investigation.

"The more the SARS-CoV-2 virus spreads, the more opportunities it has to change. High levels of transmission mean that we should expect more variants to emerge," said the WHO.

SARS-CoV-2 is the virus that causes COVID-19 disease.

Since first being reported to the WHO on December 14, the British-identified variant VOC 202012/01 has been found in 50 countries, territories, and areas, the agency said.

WHO also revealed that test results have shown that the age and sex distribution was similar to that of other circulating variants, while contact tracing data revealed "higher transmissibility (secondary attack rates) where the index case has the variant strain".

The South African-identified variant 501Y.V2, first reported on December 18, has now been detected in 20 countries, territories, and areas.

"From preliminary and ongoing investigations in South Africa, it is possible that the 501Y.V2 variant is more transmissible than variants circulating in South Africa previously," the WHO weekly report said.

"Moreover, while this new variant does not appear to cause more severe illness, the observed rapid increases in case numbers has placed health systems under pressure."

The agency further said it had been notified by Japan on January 9 of a new variant detected in four travellers arriving from Brazil. The variant was found in two adults and two children.

"This variant has 12 mutations to the spike protein, including three mutations of concern in common with VOC 202012/01 and 501Y.V2," it said, "which may impact transmissibility and host immune response".