New coronavirus restrictions have been announced after two cases of the Omicron variant were discovered on British soil.
Face coverings will now be compulsory in shops and on public transport - bringing England in line with other parts of the UK. Pubs and restaurants will be exempt.
Anyone who enters the UK from abroad will now be required to take a PCR test and self-isolate until they get a negative result.
And those who come into contact with someone who tests positive for the Omicron variant will need to self-isolate for 10 days - even if they are fully vaccinated.
At a Downing Street news conference, Boris Johnson described the measures as "temporary and precautionary" - and said they will be reviewed in three weeks' time.
There are fears that the Omicron variant, which could be more transmissible and more resistant to vaccines, may affect the plans of millions over Christmas.
While the prime minister refused to say whether further restrictions could be imposed, he said he is "confident" that this Christmas will be "considerably better than the last".
Two cases of this new strain were identified in Nottingham and Essex yesterday - and their households have been ordered to self-isolate.
Both cases have been linked to travel in southern Africa, and targeted testing is also going to take place in their surrounding areas.
The Omicron variant has also been identified in a small number of patients in Belgium, Italy, Germany and Australia.
Professor Chris Whitty said it is "inevitable" this variant will spread across the world - and it is unclear how effective current vaccines will be in protecting Britons against it.
However, he said those who are vaccinated or receive the booster jab will be less likely to become seriously ill.
The government's vaccine experts have now been tasked with determining whether booster jabs should be extended to everyone over the age of 18.
Ten countries have now been added to the red list - and from 4am today, anyone arriving from these destinations will need to quarantine in a government-approved hotel for 10 days.
MPs are expected to be given a vote to approve these new measures after they come into force, and a number of backbench Conservatives may stage a rebellion.
Mr Johnson stopped short of bringing back the work-from-home guidance or extending the use of vaccine passports.
The UK's chief scientific adviser, Sir Patrick Vallance, has warned the country may need to "face up" to the possibility of further action if the Omicron variant is very transmissible.
Sir Patrick told the Downing Street news conference: "I think we'll get more information on transmissibility, we'll get more information on the ability of the vaccines to protect against the virus, but that's going to take a little bit of time.
"At the moment, the models are more 'if it spreads very fast, of course it's going to spread very fast and go into a lot of places, and if it spreads less fast it's going to do so less'."
It could take weeks for scientists to fully understand the variant's mutations and whether existing vaccines and treatments are effective against it.
Health Secretary Sajid Javid will appear on Trevor Phillips on Sunday on Sky News at 8.30am.