The government does not want to cancel Christmas but it is “our duty” to take action when the evidence is clear, says Health Secretary Matt Hancock.
Millions of people in the UK have seen their festive plans severely restricted or scrapped after a new coronavirus variant caused cases to soar.
A stringent new lockdown has come into force in London, parts of east and south-east England and Wales.
Mr Hancock warned the new tier four rules in England could last for months.
Some 21 million people in England and Wales who entered new restrictions at midnight are being told to stay at home, while non-essential shops and businesses have to close.
The planned relaxation of rules for Christmas has been scrapped for those under England’s new toughest measures – tier four, while in the rest of England, Scotland and Wales, relaxed indoor mixing rules are cut from five days to Christmas Day only.
Mr Hancock told the BBC’s Andrew Marr programme that it was “important for everybody to act like they might have the virus”.
Asked whether the virus was under control, Hancock said: “No it’s not, the new variant is out of control and we need to bring it under control.”
He added: “Of course we don’t want to cancel Christmas… we don’t want to take any of these measures, but it’s our duty to take them when the evidence is clear.”
The health secretary said he did not know how long the tier four measures would be in place but “it may be for some time, until we can get the vaccine going”.
Transport Secretary Grant Shapps said it was very important that people followed the new travel guidance in England and “do not attempt to travel”, saying extra British Transport Police officers were being deployed.
Residents in tier four are being asked to stay at home unless for essential journeys, while people in other tiers are advised not to travel into the new tier four areas.
London Mayor Sadiq Khan told BBC Breakfast the “11th-hour announcement is a bitter blow” for families and businesses, saying it’s the “chop-change, stop-start, that’s led to so much anguish, despair, sadness and disappointment”.
“I’m afraid it makes it really difficult for people like me to ask people to listen to us when we keep on changing our minds,” he said.
However, he urged Londoners to follow the rules which he said had been brought in “for a very good reason”, adding that the NHS had told him that hospitals in London had as many Covid patients this weekend as they did at the peak of the virus in April.
Tier four restrictions:
The tier four restrictions – similar to England’s second national lockdown – applies to all areas in the South East which were in tier three, covering Kent, Buckinghamshire, Berkshire, Surrey (excluding Waverley), Gosport, Havant, Portsmouth, Rother and Hastings.
It also applies 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).
The changes for England, announced at a Downing Street briefing on Saturday, will last for two weeks with the first review due on 30 December.
In Wales, First Minister Mark Drakeford brought forward a lockdown, saying hundreds of people had already contracted the new, “more aggressive” strain of the virus there.
In Scotland, Covid restrictions will only be relaxed on Christmas Day, with mainland Scotland being placed under the tightest restrictions from Boxing Day.
A ban on travel to the rest of the UK will also apply over the festive period.
In Northern Ireland, no changes have been made to Christmas restrictions, with three households allowed to meet from 23 to 27 December. The country is set to enter a six-week lockdown from 26 December.
Scientists have warned that a new variant of the coronavirus variant is more infectious and spreading more rapidly leading Mr Johnson to say the government had to “change our method of defence”.
Susan Hopkins, from Public Health England, said they alerted the government on Friday that the new variant – first identified in the middle of October – was spreading faster than other viruses circulating.
Dr Hopkins told the BBC’s Andrew Marr programme that there was also evidence that people with the new strain had “higher viral loads” – which meant they were more infectious.
The prime minister said analysis suggested the new variant could increase the R number – which indicates if an epidemic is growing or shrinking – by 0.4 or more.
The World Health Organization has said it is in “close contact” with UK officials over the emergence of the new variant of coronavirus.
The Dutch government announced it was banning passenger flights between the Netherlands and the UK from 05:00 GMT Sunday, until 1 January at the latest, because of the new variant.
The PM’s announcement of new restrictions came just days after he defended plans to relax restrictions for five days during the festive period – despite calls by some in the medical profession to scrap the change.
Boris Johnson told the Downing Street briefing on Saturday that he knew how “disappointing” the news would be, but said he believed there was no alternative.
The steep increase in the proportion of coronavirus cases linked to this new variant is strong evidence that it is driving transmission.
In London, 28% of cases were as a result of this new mutation in mid-November, but that has now increased to more than 60%.
It may explain why, during the second lockdown, cases started to increase in London, while in Kent the tier three measures appear to have had little impact in recent weeks.
As England’s chief medical adviser Prof Chris Whitty says, this is a bad moment, but there is also some hope.
Mutations happen all the time – there have been thousands of variations to this coronavirus since it emerged – and there is nothing to suggest this causes more serious illness or will hamper the effectiveness of the vaccine.
The prime minister announced 350,000 people had been given the first dose of the vaccine in the first two weeks of the programme.
In the coming weeks, the number of GP-led vaccination clinics should increase six-fold, while approval of a second vaccine made by Oxford University could pave the way for mass vaccination centres to be set up in sports stadiums and conference centres.
That could see two million people a week being vaccinated. Within a matter of months all the over-65s could have been offered a jab. This could then start to feel very different.
But for now, the slog of the pandemic continues – and for many it just got harder.
Labour leader Sir Keir Starmer, who raised the prospect of tougher restrictions in the Commons on Wednesday, said he was “really frustrated”.
“Millions of families will be heartbroken by having Christmas plans ripped up,” Sir Keir said.
Mr Johnson is also facing criticism from within Conservative Party ranks.
Mark Harper, chairman of the Covid Recovery Group of Tory MPs – which has been highly critical of the government’s strategy – called for Parliament to be recalled so MPs could debate and vote on the changes.
British Chambers of Commerce director general Adam Marshall asked what support would be available for companies whose cash flow projections “have once again been thrown into chaos”.
In other developments: