A worrying new Covid variant could already be in the UK, a health chief has admitted.
Scientists have so far only detected the B.1.1.529 variant in relatively small numbers in South Africa, Botswana and Hong Kong, but they are concerned by its high number of mutations, which could help it evade the body's immune response and make it more transmissible.
UK Health Security Agency chief medical adviser Dr Susan Hopkins admitted that the new variant was “the most worrying we've seen".
Asked if it could already be in Britain, she told the BBC : “It’s always possible.”
But she added: “We have no cases identified whatsoever yet.”
Flights to the UK are being suspended from six southern African countries - South Africa, Namibia, Botswana, Zimbabwe, Lesotho and Eswatini - to prevent the spread of the new variant.
From 4am on Sunday all travellers from these destinations will have to spend 10 days in quarantine hotels.
British health officials are scrambling to trace anyone who has returned from these countries in recent days to see if they are carrying the new strain, which is expected to be named 'Nu'.
Dr Hopkins said transmission rate of the new variant was "really quite high".
The R rate in Gauteng, where the new variant was found in South Africa, is around 2 - which higher than its been in the UK since the start of the pandemic, she said.
Transport Secretary Grant Shapps warned: “It is inevitable, of course, that it will go all around the world if it is going to do so.
"This doesn't prevent it from coming here but it gives us a chance to grow the cultures and test those questions about vaccines and treatments against it.”
He feared the new variant could "defeat the vaccine" but insisted that ministers acted "extremely fast" to ensure a "safety-first approach" to travel changes following an emergency meeting with chief medical officers.
"This is agreed across all of the United Kingdom and we have acted fast, it's rather like the mink variant from Denmark last year, where we acted very quickly, within hours and we're then able, once we've checked it out, to release things somewhat,” said Mr Shapps.
"I hope that's what this is, a pause rather than going backwards, but we can't take risks when we see a variant which could well defeat the vaccine, or at least that's the concern and we need just a bit of time to check that out."
