Households in London will be banned from mixing over the festive period, while in the rest of the country, three households will be able to meet indoors on Christmas Day only.

Giving an update from Downing Street, the prime minister also said London and the south-east will move into a tier 4 from midnight, with lockdown restrictions coming into force.

Speaking alongside Chief Medical Officer Professor Chris Whitty and Chief Scientific Advisor Sir Patrick Vallance, Boris Johnson said the tightening of the restrictions is linked to the spread of a mutant variant of Covid-19 which scientists believe is spreading more quickly than the previous form.

This new variant is most prevalent in the south-east of England.

Under the new tier 4 restrictions, a stay at home message will be put into law with exemptions for education, childcare and exercise.

All non-essential retail, indoor leisure, entertainment and beauty care (such as hairdressers and nail bars) will be forced to close. 

Indoor hospitality, such as pubs, restaurants and cafes are already shut in these areas as they are in tier 3.

The following areas will be under tier 4:

  • Kent
  • Buckinghamshire
  • Berkshire
  • Surrey (excluding Waverley)
  • The boroughs of Gosport, Havant, Portsmouth, Rother and Hastings
  • All 32 London boroughs and the city of London.
  • Bedford
  • Central Bedfordshire
  • Milton Keynes
  • Luton
  • Peterborough
  • Hertfordshire
  • Essex (excluding Colchester, Uttlesford and Tendring)

The new rules for tier 4 which come into force from midnight are:

People are advised not to enter tier 4 areas and residents in tier must not stay overnight away from home

You should work from home. Where you cannot, you are still able to travel to work

Non-essential shops will be shut

Indoor leisure such as gyms and swimming pools will be shut

Indoor entertainment such as cinemas and bowling alleys will be shut

Beauty services, such as hairdressers and nail bars will be shut

One person can meet with one person from another household outdoors

Support bubbles and childcare bubbles are exempt from the rules, and people are allowed outdoors for unlimited exercise.

The rules will be in place for two weeks and then will be reviewed.

The introduction of tier 4 and severe restrictions around Christmas come amid soaring coronavirus cases, particularly in London and south-east England where a new Covid variant has been identified.

On Saturday, a further 534 people died with 28 days of a coronavirus test while a further 27,052 people tested positive for coronavirus, bringing the UK total since the start of the pandemic to more than two million cases.

This new variant is understood to be much more transmissible than the form of coronavirus we have been leaving with for months.

Weekly average case rate has increased by more than 66% compared to two weeks previously. In London, this has almost doubled in the last week and the new variant accounts for 60% of all new infections in London. 

Speaking ahead of the Downing Street briefing, a spokesperson for the prime minister said that during November’s lockdown, the number of Covid cases in Kent did not fall and they are continuing to rise with the county under tier 3 restrictions, indicating that tier 3 measures are not strong enough to contain the spread of the virus and that a new fourth tier is required where the prevalence of the respiratory disease is highest.

Speaking earlier on Saturday, Chief Medical Officer Professor Chris Whitty said that while new Covid variant spreads "more quickly" than the type we have been living with for months, "there is no current evidence to suggest the new strain causes a higher mortality rate or that it affects vaccines and treatments" but further study would be needed to assess this.

The Downing Street update came after Mr Johnson held a Cabinet meeting to discuss the impact of the new Covid strain.

He also spoke to Labour leader Sir Keir Starmer and the leaders of Wales, Northern Ireland and Scotland.