A worrying new variant of coronavirus may already be in the UK although current signs are reassuring, an expert at the Health Security Agency (UKHSA) has said.

Dr Susan Hopkins, chief medical adviser of the UKHSA, said scientists are very concerned about the new variant found in South Africa, Botswana, Hong Kong and Israel. No cases have been detected in the UK.

Transport Secretary Grant Shapps said the UK was “buying time” by adding South Africa, Botswana, Lesotho, Eswatini, Zimbabwe and Namibia to its travel red list, adding that the Government was taking a “safety-first approach”.

Passengers arriving in the UK from these countries from 4am on Sunday will be required to book and pay for a Government-approved hotel quarantine for 10 days. Downing Street urged anyone who has arrived from these countries recently to get tested.

On Friday, Health Secretary Sajid Javid told the Commons discussions are ongoing over the prospect of adding further countries to the red list as he said there is “huge international concern” over the variant.

He said: “We are keeping this under review and there’s very live discussions going on about whether we should and when we might add further countries, and we won’t hesitate to act if we need to do so.”

Mr Javid said experience has shown “we must move quickly and at the earliest possible moment”, adding there were concerns the variant may be more transmissible, make vaccines less effective and may affect one of the UK’s Covid treatments, Ronapreve.

He told the House of Commons it was “highly likely” the B.1.1.529 variant had already spread from South Africa and Botswana, where confirmed cases have been found, to other countries.

It's a highly complex mutation, there's also new ones that we have never seen before

Dr Hopkins told BBC Radio 4’s Today programme that the new variant had around “30 different mutations that seem relevant – that’s double what we had in Delta (variant)”.

She added: “If we look at those mutations, there’s mutations that increase infectivity, mutations that evade the immune response both from vaccines and from natural immunity, mutations that cause increased transmissibility.

“It’s a highly complex mutation, there’s also new ones that we have never seen before.”

She said the variant was the “most worrying” seen by scientists but much was still unknown.

South African scientists raised concerns over the variant, which they fear is behind a spike in cases in some regions, including Gauteng province, which includes the cities of Pretoria and Johannesburg.

“What we’re seeing in South Africa is that they were at a very, very low point with very low amount of cases being detected a day,” Dr Hopkins said.

“In a shorter period than two weeks, they have more than doubled their epidemiology picture, they are saying that the transmission rates, the R value that they have in Gauteng – around where this was first found – is now 2, which is really quite high.

“We haven’t seen levels of transmission like that since right back at the beginning of the pandemic… so that would cause a major problem if you had that high transmission with this type of virus in a population where it may evade the immune responses that are already there.”

She said South Africa was not a highly vaccinated population but it was “highly immune” due to prior infection.

Work is continuing to see whether the new variant may be causing new infection in people who have already had coronavirus or a vaccine, or whether waning immunity may be playing a role.

Asked if the variant may already be in the UK, she said: “Well, it’s always possible. We have no cases identified whatsoever yet, nothing in our genome sequencing… so overall, I think the situation is reassuring in-country, but of course, people are arriving every day.”

She added that one of the mutations in the variant is very similar to one in Alpha, which means it can be detected quite easily with PCR tests.

Asked on ITV’s Good Morning Britain how likely it was the variant would come to the UK, Dr Hopkins said: “I think the measures that we take on borders are there to delay, to slow, to allow us to have time to gain more information.

“The fact this virus is resurging on a very immune population (in South Africa) is worrying.

“Clearly vaccines will help, because they will boost the antibody responses and the T cell responses in our body, which will help us fight variants, so global vaccination is a key component of that.

“But also we need to watch and wait and see as these new variants emerge. And I think this shows that we may need new vaccines in the future.”

Mr Shapps said the Government acted as soon as the UK’s chief medical officers came to ministers following their meeting at around 4pm on Thursday.

He told the Today programme: “I think all the history of coronavirus suggests that it is best to act quickly, determine the extent of the way that the virus interacts with vaccines, treatments, transmissibility and then give yourself a bit more time.

“It is inevitable, of course, that it will go all around the world if it is going to do so.

“So this doesn’t prevent it from coming here, but it slows things up and gives us the chance to grow the cultures and test those questions about vaccines and treatments against it.”

Asked if the travel restrictions were about “buying time”, he said: “Yeah, that’s spot on. It’s about buying time.”

Other countries including Germany, Italy, France, Israel and Singapore have also restricted travel over the variant, and European Commission president Ursula von der Leyen has proposed stopping air travel from the southern African region.

However, South Africa’s foreign minister Naledi Pandor said on Friday that the UK’s decision to ban flights “seems to have been rushed”.

She said: “Whilst South Africa respects the right of all countries to take the necessary precautionary measures to protect their citizens, the UK’s decision to temporarily ban South Africans from entering the UK seems to have been rushed as even the World Health Organisation is yet to advise on the next steps.”

Professor Glenda Gray, president and chief executive of the South African Medical Research Council, told the Today programme the variant was showing between 26 and 32 changes in the spike protein of the virus and 45 amino acid changes.

She added: “We are entering into the fourth wave, and whether it’s driven by this new variant is of high concern because it may mean that people who have been infected with Delta or other variants may not be protected this time around.”

Professor James Naismith, director of the Rosalind Franklin Institute in Oxford, told the programme the new variant will “almost certainly” make vaccines less effective, though they would still offer protection.

He added: “If it spreads more quickly then yes, it will get here. The travel ban will delay its arrival but if it spreads more quickly the lesson has surely been from all the variants we’ve seen before that it will get here eventually.”