Boris Johnson has issued a new “stay at home” order covering London and much of the south and east of England, and dramatically curtailed plans for Christmas mixing nationwide, in response to a new fast-spreading strain of coronavirus. Just days after reaffirming the festive “bubble” plan and claiming it would be “inhuman” to cancel Christmas, the prime minister announced the creation of a new, stricter “tier 4”. Only households living outside this area will now be allowed to mix for Christmas in England – and then only for a single day, 25 December, with long-distance travel discouraged. “It is with a very heavy heart that I must tell you we cannot continue with Christmas as planned,” the prime minister said, insisting there was no alternative. “I know how much emotion people invest in this time of year, and how important it is, for instance for grandparents to see their grandchildren, for families to be together,” he said. “We have said throughout this pandemic that we must and we will be guided by the science. When the science changes, we must change our response.” The Scottish and Welsh governments have been briefed on the situation and have announced their own plans. The new tier 4 rules will come into effect at midnight, and will be similar to those in place during the four-week lockdown in November. Non-essential retail, leisure venues and close-contact services such as hairdressers will not be allowed to reopen after they close on Saturday. Members of the public will be ordered to stay at home, aside from a few exemptions such as taking exercise. The only mixing with other households allowed in tier 4 will be meeting up with a single other person outdoors. The area affected includes all of London and much of the south and east of England, including Kent, most of Essex, most of Surrey, as well Buckinghamshire, Berkshire, Hertfordshire, parts of Bedfordshire, Gosport, Portsmouth and Peterborough. The prime minister said travel was not recommended anywhere in England. “We’re asking everyone, in all tiers, to stay local,” he said. The restrictions were announced by the prime minister at a hastily convened press conference on Saturday, alongside the chief medical officer, Chris Whitty, and the chief scientific adviser, Patrick Vallance. They will initially be in place for a fortnight. Asked whether existing guidance such as the 2-metre distancing rule may need to be changed, Whitty said the same measures would still be effective, but, “we’re going to have to do a lot more to stand still”. Johnson announced the draconian new restrictions after being presented with evidence on Friday night that the new strain of coronavirus recently identified in the capital and in Kent is spreading more rapidly than the previous strain. Johnson said the government’s New and Emerging Respiratory Virus Threats Advisory Group (Nervtag) had estimated that the new variant may be up to 70% more transmissable. Scientists do not yet have evidence that the new variant has a higher mortality rate, or that it affects the effectiveness of vaccines or treatments for Covid. But the government’s scientific advisers have become increasingly alarmed about evidence and modelling suggesting it is spreading very rapidly. In particular, there were concerns that the prevalence of the virus had not been falling in Kent, despite the county being under tier 3 restrictions. The weekly average case rate for England has increased by more than 66% in the past fortnight, and in London it has nearly doubled in the past week. The new variant is accounting for 60% of infections in London. The plans were confirmed in a cabinet call on Saturday afternoon, and the prime minister also spoke to the Labour leader, Keir Starmer, to discuss them. Just three days ago, Johnson taunted Starmer with wanting to “cancel Christmas”, as he insisted the government would be sticking with looser regulations allowing up to three families to mix for up to five days. Instead of changing the rules, the prime minister had exhorted the public to be cautious, saying, “a smaller Christmas is going to be a safer Christmas, and shorter Christmas is a safer Christmas”. The mayor of London, Sadiq Khan, said the announcement was a “bitter blow for Londoners who were hoping to spend time safely with loved ones this Christmas”. He called on the government to “vastly increase mass testing,” and to increase financial support to business and the self-employed hit by the restrictions.