London and parts of Essex and Hertfordshire will move into the highest tier of coronavirus restrictions at midnight on Tuesday, health secretary Matt Hancock has confirmed.
The change is due to a surge in coronavirus cases in these areas, he said in the House of Commons.
He also said experts had identified a new variant of coronavirus that “may be associated with the faster spread in the South of England”.
London is currently in Tier 2 following the month-long England-wide lockdown, which ended on 2 December.
From Wednesday, pubs and restaurants will close, except for takeaway and delivery services.
The next review of the tier system will be on 23 December.
Watch: London mayor Sadiq Khan calls for schools to close early
The latest figures showed nearly 24,000 confirmed cases in London in a week, with cases rising across all boroughs.
The escalating epidemic was discussed by the COVID “O” committee, chaired by Boris Johnson, on Monday morning.
London mayor Sadiq Khan said the rise in cases was “deeply concerning” and urged the government to provide additional support to businesses and those who cannot work from home.
He told Sky News: “I am calling on the government to urgently provide additional support to get the spread under control, save lives and livelihoods and ensure our NHS is not overwhelmed this winter.
“Increased testing is key to this, which is why I want to see regular asymptomatic testing extended to all those unable to work from home and to students and staff at London's secondary schools, sixth-form college and FE (further education) colleges.”
He has also called for schools to close early and to reopen later in January amid a “significant” rise in infections among 10- to 19-year-olds.
Although rules are getting tighter in many areas of the UK, the easing of restrictions over Christmas will still go ahead.
Families will be allowed to form a limited Christmas bubble from 23-27 December of people from no more than three households.
The prime minister’s official spokesperson said there were no plans to shorten the period when families can meet up in their Christmas bubbles.
What are the rules in Tier 3?
People are banned from socialising indoors with people they don’t live with
People are allowed to meet in groups of up to six in some outdoor public spaces such as parks, beaches, public gardens, heritage sites or castles and sports facilities
Bars and restaurants must close unless they can offer takeaway
Accommodation such as hotels, bed and breakfasts, campsites and guest houses must close
Indoor entertainment venues, from casinos to cinemas, must close
Indoor attractions at mostly outdoor venues must close, including indoor attractions within zoos, safari parks, wildlife reserves, aquariums, farms and theme parks
Leisure and sports facilities can continue to stay open, but without group exercise classes
Attendance at shows, performances or sporting events is not allowed
People can attend places of worship but, while inside, are not allowed to socialise with others they don’t live with
Up to 15 people can attend wedding ceremonies but receptions aren’t permitted. Up to 30 can attend funerals
Organised outdoor sport, physical activity or exercise classes can continue, though “higher-risk contact activity” should not take place. None of these activities can happen indoors
Read more: The Tier 2 COVID lockdown rules explained
The following venues are allowed to open in all three tiers:
Non-essential retail shops, as well as indoor and outdoor markets
Leisure centres, gyms, dance studios, golf courses, swimming pools, riding centres, outdoor playgrounds
Close-contact personal care shops such as barbers and hairdressers, beauty salons, tattoo parlours, nail salons, spas and beauty services, massage parlours and tanning salons
Public buildings such as libraries and community centres
Watch: The COVID dos and don’ts of Christmas this year