Boris Johnson has cancelled Christmas for almost 18 million people across London and eastern and south-east England, after scientists warned of the rapid spread of the new variant coronavirus.

The Prime Minister announced that from Sunday areas in the South East currently in Tier 3 will be moved into a new Tier 4 for two weeks – effectively returning to the lockdown rules of November.

In the rest of England, the planned Christmas easing of the restrictions – allowing three households to meet over the festive period – will be severely curtailed to apply to Christmas Day only.

At a No 10 news conference, Mr Johnson said he was taking the actions with a “heavy heart”, but the scientific evidence – suggesting the new strain was up to 70% more transmissible – had left him with no choice.

“Without action the evidence suggests that infections would soar, hospitals would become overwhelmed and many thousands more would lose their lives,” he said.

“Yes Christmas this year will be different, very different. We’re sacrificing the chance to see our loved ones this Christmas so that we have a better chance of protecting their lives, so that we can see them at future Christmases.”

Following his announcement, Welsh First Minister Mark Drakeford said he was bringing forward alert Level four restrictions for Wales from Sunday – in line with the Tier 4 rules in England, while Christmas “bubbles” would be confined to Christmas Day only.

“The situation is incredibly serious. I cannot overstate this,” he said. “We cannot expose people to the risk of this new, more virulent strain of coronavirus.”

Scotland’s First Minister Nicola Sturgeon said the strict travel ban between Scotland and the rest of the UK would now remain in place right throughout the festive period.

The announcements came however as a bitter blow to millions of families looking forward to spending time together, and to retailers hoping to make up for a torrid year with last minute Christmas sales.

British Chambers of Commerce director general Adam Marshall called for urgent Government support for businesses now facing closure.

“Christmas was already cancelled for many businesses, but even more will now suffer as a result of this last-minute decision,” he said.

“While Government must act on public health concerns, it must also address the economic consequences of its actions.”

Mark Harper, the leader of the Covid Recovery Group of Tory MPs critical of the Government’s response to the pandemic, demanded the recall of Parliament so MPs could debate and vote on the changes for England.

“Given the three-tier system and the initial Christmas household rules were expressly authorised by the House of Commons, these changes must also be put to a vote in the Commons at the earliest opportunity, even if that means a recall of the House, ” he said.

British Medical Association council chair Dr Chaand Nagpaul however welcomed the announcement which, he said, would save lives.

“These tougher measures are necessary to protect our loved ones and those most vulnerable in our society, as well as giving our health service a fighting chance to cope with the incredible demand it is experiencing,” he said.

For those areas affected, non-essential shops, gyms, cinemas, hairdressers and bowling alleys will be forced to close – while people will be restricted to meeting one other person from another household in an outdoor public space.

This new variant not only moves fast but it is becoming the dominant variant. It is beating all the others in terms of transmission

There will be a review of the rules in England on December 30.The dramatic move came after scientists on the Government’s New and Emerging Respiratory Virus Threats Advisory Group (Nervtag) concluded the mutant strain identified by the Public Health England laboratories at Porton Down was spreading more quickly.

The Prime Minister was advised of the group’s conclusions at a meeting with ministers on the Covid O Committee on Friday evening, and the new regulations were signed off by the Cabinet in a conference call on Saturday lunchtime.

Sir Patrick Vallance, the Government’s chief scientific adviser, said the new variant, known as VUI 2020/01, was thought to have originated in either London or Kent in September.

By November, it was accounting for 28% of new infections in the region and by early December that had risen to 60%.

“This new variant not only moves fast but it is becoming the dominant variant. It is beating all the others in terms of transmission,” he said.

He said however there was no evidence it causes a more severe illness than the original virus, while the “working assumption ” of scientists was that the vaccines that had been developed should be able to deal with it.The Government’s Scientific Advisory Group for Emergencies (Sage) warned that without action the new variant could add 0.4 to the R number – representing the reproduction rate – which was already estimated at 1.1. to 1.2.

Under the new “stay at home” order – covering around a third of the population of England – people in Tier 4 will be told they should not stay away from home overnight and people from outside will be advised not to visit Tier 4 areas.

People throughout England are advised to “stay local” and avoid overseas travel, while those in Tier 4 will only be able to go abroad for essential business.

The new Tier 4 restrictions will apply in all Tier 3 areas in the South East – covering Kent, Buckinghamshire, Berkshire, Surrey (excluding Waverley), Gosport, Havant, Portsmouth, Rother and Hastings.

It will also apply in London (all 32 boroughs and the City of London) and the East of England – Bedford, Central Bedford, Milton Keynes, Luton, Peterborough, Hertfordshire and Essex (excluding Colchester, Uttlesford and Tendring).