The delta variant is of greater concern to the UK than the new Covid mutation, England's chief medical officer has said amid rising panic about the omicron strain.
Prof Chris Whitty said ministers were right to take "precautionary" border measures to slow the arrival of the new variant into Britain.
But he warned that any attempt to impose "more muscular restrictions" could lose public support and suggested concern was best focused on more immediate threats.
His comments came amid rising fear around the world, with billions wiped off shares. London's FTSE 100 fell 3.6 per cent – its biggest slide in more than a year – as investors took fright, wiping £72 billion off the value of Britain's blue chip stocks.
IAG, the British Airways owner, was the biggest victim as the UK and Europe rushed to impose travel restrictions on South Africa – where the variant was first identified – and other southern African nations. Shares dropped almost 15 per cent.
Stock markets in Europe suffered even worse damage as soaring virus case numbers and low vaccination rates fuelled fears that other nations will be forced to follow Austria into full lockdown.
Germany's Dax closed down 4.2 per cent, while the Cac 40 in Paris was the worst performer among the major markets, plummeting 4.8 per cent.
Prof James Naismith, the director of the Rosalind Franklin Institute, told BBC Radio 4's Today programme that vaccines and new treatments for Covid meant the developments were "bad news but not doomsday".
On Friday night, the US became the latest country to impose travel restrictions, with arrivals from South Africa, Botswana, Zimbabwe, Namibia, Lesotho, Eswatini (the former Swaziland), Mozambique and Malawi mostly banned from Monday.
Experts said South Africa could be faring worse because of low vaccine uptake – currently, only 24 per cent of citizens have been fully jabbed. Although little is currently known about the vaccination status of the South African cases, the infected person in Belgium was unvaccinated.
Cases have yet to be detected in Britain, but scientists said the border controls which began to be imposed on Friday were only likely to "buy time" before it arrived.
Government advisers called for new domestic restrictions to be brought in to help limit any potential spread.
Prof Sharon Peacock, the director of COG-UK, which monitors new variants for the Government, said: "There are two approaches to what happens next: wait for more scientific evidence, or act now and row back later if it wasn't required.
"I believe that it is better to go hard, go early and go fast and apologise if mistaken than to take an academic view that we need to reach a tipping point in evidence before action is taken.
"There are sufficient red flags to assume the worst rather than hope for the best, and to take a precautionary approach."
Prof Susan Michie, from the Scientific Advisory Group for Emergencies (Sage), called for immediate restrictions, including encouraging everyone to work from home where possible and wear face masks indoors.
Asked whether "Plan B" restrictions should be enacted before the new variant got a foothold in Britain, Prof John Edmunds, of the London School of Hygiene and Tropical Medicine, said: "It always helps to get on top of things early."
Sajid Javid, the Health Secretary, confirmed on Friday that the Government was still following "Plan A" for managing Covid this autumn and winter but warned: "If we need to go further, we will."
He told the Commons the Government would "continue do whatever is necessary to keep us safe" as the world faced a variant of "huge international concern".
Mr Javid said the new variant "may pose a substantial risk to public health" and that "early indications show this variant may be more transmissible than the delta variant and current vaccines may be less effective against it".
A senior government source said the action to put six countries on the red list came from "an abundance of caution" as it would take a couple of weeks to measure vaccine efficacy. The move, which left hundreds of British holidaymakers scrambling to get home, will force others to cancel Christmas trips.
On Friday, the World Health Organisation , with cases identified in Israel and Belgium. Scientists fear the variant's high number of mutations could mean that it can evade vaccines.
However, other experts sought to reassure the public, with one Oxford scientist saying the new variant was "not doomsday".
Prof Whitty said he was more concerned about the risks posed by existing variants, describing the delta epidemic as "undoubtedly the principal thing we need to concern ourselves with between now and Christmas".
He told a Local Government Association panel discussion: "We've always known that new variants would crop up from time to time … but there's an awful lot we don't know and I think it's probably not terribly helpful to speculate."
Amid calls from some scientists for Britain to embark on the Government's "plan B" winter restrictions, Prof Whitty said: "My greatest worry at the moment is that if we need to do something more muscular at some point, whether it's for the current new variant or at some later stage, can we still take people with us?"
Sir John Bell, one of the Government's most senior advisers on vaccines, said the new variant may end up causing no more than "runny noses and headaches" in those who have been vaccinated.
Sir John, the regius professor of medicine at the University of Oxford, said that while the new variant might evade antibodies, it would be less likely to escape T-cells and other parts of the immune system that provide broader protection.
"You could still have a highly infectious virus that scoots around and causes lots of trouble, but causes lots of, you know, runny noses and headaches but doesn't put people into hospital. Honestly, you could live with that, I think," he said.
On Friday, Dr Angelique Coetzee, who chairs the South African Medical Association, described the travel restrictions imposed on the country as "hasty" and the reaction from other countries as "a storm in a teacup". Dr Coetzee asked why everyone was "up in arms" and said: "So far, what we have seen is very mild cases."