A new strain of coronavirus has been identified in southern England as tougher restrictions were imposed in London.
The capital and parts of Essex and Hertfordshire will face Tier 3 restrictions from Wednesday following "very sharp, exponential rises" in cases.
Health Secretary Matt Hancock told MPs the number of cases involving the new variant was "increasing rapidly".
Initial analysis suggests that this variant is growing faster than the existing variants, he said.
"We've currently identified over 1,000 cases with this variant predominantly in the south of England although cases have been identified in nearly 60 different local authority areas."
The Health Secretary said there was nothing to suggest the variant was more likely to cause serious disease and he suggested the vaccine should still be effective.
"But it shows we've got to be vigilant and follow the rules and everyone needs to take personal responsibility not to spread this virus," he said.
Mr Hancock said it was necessary to move London from Tier 2 to Tier 3 to prevent the NHS being overwhelmed.
"Over the last week we have seen very sharp, exponential rises in the virus across London, Kent, parts of Essex and Hertfordshire," he said.
"We do not know the extent to which this is because of the new variant but no matter its cause we have to take swift and decisive action which unfortunately is absolutely essential to control this deadly disease while the vaccine is rolled out.
"In some parts of these areas the doubling time is around every seven days."
Hospitals across the capital, Essex and Kent were already "under pressure", he warned.
"We know that this doubling of cases will be mirrored in hospital admissions and it only takes a few doublings for the NHS to be overwhelmed."
Areas moving to Tier 3 are: Greater London; south and west Essex including Basildon, Brentwood, Harlow, Epping Forest, Castle Point, Rochford, Malden, Braintree and Chelmsford along with Thurrock and Southend-on-Sea borough councils; and the south of Hertfordshire including Broxbourne, Hertsmere, Watford and the Three Rivers local authority.
As well as the restrictions on hospitality, the move means people should avoid travelling into or out of the area – potentially depriving Oxford Street and London's other shopping centres of much-needed custom in the run-up to Christmas.
Mr Hancock said: "I know that this is difficult news and I know that it will mean plans disrupted and for businesses affected it will be a significant blow.
"But this action is absolutely essential not just to keep people safe but because we have seen that early action can help prevent more damaging and longer-lasting problems later."
London Mayor Sadiq Khan said the decision was "incredibly disappointing" for businesses but urged the capital's residents to follow the rules.
He added: "The worst thing for London's businesses and our economy would be yet another full lockdown in the new year.
"That's why I urge Londoners to follow the Tier 3 rules that the Government is putting in place very closely so that we can drive down infection rates as much as possible."
Mr Hancock faced criticism from Tories over the move.
Former Conservative leader Sir Iain Duncan Smith, MP for Chingford and Woodford Green, said hospitality venues had worked to control the virus, urging Mr Hancock to "seek some kind of flexibility within this so it targets better the real risk and doesn't just hammer those that have been doing the right thing".
Former minister Robert Halfon, MP for Harlow, said hospitality businesses are "on their knees".
Cities of London and Westminster MP Nickie Aiken accepted the measures were necessary but they "will have devastating economic consequences".
The worrying rise in cases came as GP surgeries in England began vaccinating people against coronavirus and Mr Hancock urged people not to "waver as we enter the final stretch" of the fight against Covid-19.
GP practices in more than 100 locations were administering the Pfizer/BioNTech vaccine, with the over-80s among those called up to receive the jab.
Care home residents in Scotland began to receive the Pfizer vaccine on Monday, while those in England's care homes can expect to see roving teams administer the jab from later this week.
Dr Nikita Kanani, director of primary care at NHS England, urged all those expecting to receive the vaccine to be patient and wait to be called up by their GP.
"There's a huge range of things that general practices are already doing so if we can ask for people to just wait a moment and wait to be contacted that would be very appreciated," she told BBC Radio 4's Today programme.
The vaccination centres will operate from doctors' surgeries or community hubs in villages, towns and cities, and come after more than 70 UK hospital hubs began administering jabs.