Boris Johnson is facing intense pressure to impose another national lockdown within days, as more than 40 countries banned arrivals from the UK in an effort to keep out a new fast-spreading variant of coronavirus. Government scientific advisers warned that inaction could cost tens of thousands of lives and risk an “economic, human and social disaster”, with the new strain spreading across the UK and overseas. Ministers urged people to avoid panic-buying food as France failed to lift its ban on freight and passengers from the UK on Monday despite a personal appeal from the prime minister, who asked Emmanuel Macron to put aside his “anxiety” over the new strain of Covid-19. Speaking at a Downing Street press conference, Johnson strived to reassure other world leaders that Britain had responded swiftly. “I hope that everybody can see that as soon as we were briefed as a government of the fast transmissibility of this new strain … we lodged all the necessary information with the World Health Organization. And we took prompt and decisive action the very next day to curb the spread of the variant within the UK,” he said. Johnson said delays to cross-Channel freight affected a small proportion of food entering the UK. “As British supermarkets have said, their supply chains are strong and robust, so everyone can continue to shop normally,” he said. Even so, some shoppers reported queues and empty shelves in supermarkets. Downing Street insisted the measures imposed at the weekend, which placed London and areas in the south and east of England in a lockdown-like tier 4 but left people elsewhere still able to mix on Christmas Day, could control the spread of the variant. But the message from health officials and scientists was very different. In a joint statement, Greater Manchester’s directors of public health said anyone arriving in the region from a tier 4 area or from Wales, which is also in effect in lockdown, should self-isolate for at least 10 days. Cases of the Covid mutation, which is thought to spread between people with up to 70% greater efficiency, have been detected across the UK, and Johnson’s chief scientific adviser said stronger curbs were likely. “The lesson I think you have to learn about this virus … is that it’s important to get ahead of it in terms of actions,” Sir Patrick Vallance told a Downing Street press conference alongside Johnson. “I think it is likely that this will grow in numbers of the variant across the country, and I think it’s likely therefore that measures will need to be increased in some places in due course, not reduced.” Since news emerged at the weekend about the rapid spread of the new variant across parts of England, at least 42 countries have banned all flights from the UK or said that only their nationals may be allowed in following a negative coronavirus test. They include: Belgium, Italy, Austria, Germany, Spain, Portugal, Bulgaria, Hungary, Luxembourg, Slovakia, Poland, Romania, Switzerland, South Africa, Lithuania, Latvia, Estonia, Croatia, France, Malta, Sweden, Turkey, Hong Kong, Canada, India, Russia, the Netherlands, Denmark, Norway, Israel, Saudi Arabia, Oman, Kuwait, Jordan, El Salvador, Ireland, the Czech Republic, Colombia, Morocco, Chile, Finland and Argentina. In a briefing on Monday, scientists said they had examined additional data and now had “high confidence” that the new strain had a transmission advantage over other virus variants circulating in the UK. “We still don’t understand the exact biological mechanisms,” said Peter Horby, a professor of emerging infectious diseases at Oxford University who chairs the government’s New and Emerging Respiratory Virus Threats Advisory Group (Nervtag). “So there’s still a lot of uncertainty about exactly how it’s occurring, and exactly the extent of the extra transmissibility.” Other scientists said the variant was unlikely to be largely contained to south and east of England, and noted that the UK was heading towards the first-wave peak of hospital admissions and deaths, to accompany the added toll of winter on the NHS. A further 33,364 people were reported to have tested positive UK-wide on Monday and 215 people died. As Johnson confirmed that more than 500,000 people had received the first dose of the only approved Covid vaccine, scientists said harsher measures now could save the lives of those yet to be immunised. Andrew Hayward, a professor of infectious disease epidemiology at University College London and a member of the government’s Scientific Advisory Group for Emergencies (Sage), said the new variant had the potential to spread around the UK and overseas. “I recognise that we have restricted travel from the tier 4 areas … but this transmission is not only in those tier 4 areas, it’s there at some level across the country,” he said. “We’re just entering a really critical phase of this pandemic, and it makes absolute sense … to act decisively I would say across the country, as many other countries have done, despite them not as far as we know being affected by this strain.” Asked if it would advisable to have a national lockdown, Hayward said: “Personally, I think it’s clearer to give a consistent national message because although the levels of risk are different in different parts of the country, they’re still there and they’re still substantial. “Essentially what we’re really doing is very much trying to reduce transmission until we can protect as many of the vulnerable as possible and that would buy us many more weeks, in which we could get people vaccinated. I couldn’t give you a precise estimate … but my view is it will be in the scale of tens of thousands of lives. “An increase in transmissibility, even given exactly the same mortality rate per person, will lead to many, many more deaths unless one increases the effectiveness of our control measures.” Robert West, a professor of health psychology at University College London’s Institute of Epidemiology and Health, who sits on the advisory group on behavioural science for Sage, said his personal opinion was that the current system was “unlikely” to contain the spread of the virus. He said: “We need to reset our strategy and move rapidly to a zero Covid strategy of the kind that many have been proposing. This will involve stricter but more rational social distancing rules across the country, and finally doing what we should have done from the start – to build the kind of test, travel, isolate and support programmes they have in countries in the far east. “It sounds expensive but the alternative could well be a catastrophic collapse in confidence in the country’s ability to control the virus and the economic, human and social disaster that would follow.” Asked why tier 4 measures were not being pushed out across England, Johnson’s spokesman said: “I would point to the fact that we’ve been clear that people shouldn’t be travelling out of tier 4 areas, and I would point to the changes that we have made in terms of tier 4,” he said. “We’re being clear that people should remain at home and limit human interaction.” Asked whether parliament could be recalled to allow angry MPs to debate the new measures, the spokesman said this would only be done to ratify any Brexit deal. “If we need to recall parliament for the purposes of Brexit, we will.”