Britain is to test travellers from South Africa – and could bring in travel restrictions – over a new COVID variant which UK experts say is the “worst one we’ve seen so far”.

The UK Health Security Agency (UKHSA) has been advising ministers, with the government understood to be considering restrictions between the UK and South Africa, where cases are rapidly rising.

The UKHSA says B.1.1.529 has a spike protein which is “dramatically” different to the original COVID strain, and it has double the amount of mutations of the Delta variant.

These mutations are likely to evade the immune response generated by prior infection and vaccination, they say.

Tom Peacock, a virologist at Imperial College London, branded the mutations “really awful” but said cases were currently “super low”.

Spike proteins are what viruses use to get into human cells, and some vaccines work by training the body to recognise the spikes and neutralise them.

Mutations on the spike could therefore potentially prove problematic.

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But with only a handful of recorded cases – three in Botswana, around 53 in South Africa and one in Hong Kong from someone who travelled from South Africa – scientists are hopeful it can be contained.

Francois Balloux, professor of computational systems biology at University College London, said it should be closely monitored but “there is no reason to get overly concerned, unless it starts going up in frequency in the near future”.

World Health Organisation’s experts are meeting on Friday to assess the variant, which on Wednesday was classed a variant under monitoring (VUM).

If it’s upgraded to a variant of concern (VOC) it could be given a name from the Greek alphabet.

However, it could also be classed as a less serious variant of interest (VOI), indicating it has characteristics that may affect factors such as transmissibility and disease severity.

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