Boris Johnson was joined by chief medical officer Professor Chris Whitty and Sir Patrick Vallance, the chief scientific adviser, for a Downing Street press conference on Saturday following the emergence of the Omicron coronavirus variant in the UK.
Mr Johnson has ordered the return of mandatory mask-wearing in shops and on public transport, and for contacts of Omicron cases to self-isolate, even if they are vaccinated, after the concerning variant was detected in Britain.
All international arrivals will have to take a PCR test by the end of their second day in the UK, as the Prime Minister prepared to reimpose measures to control the spread of coronavirus over fears the new strain could evade existing vaccines.
The Prime Minister said the â€œtemporary and precautionaryâ€ measures will be reviewed in three weeks, while the Governmentâ€™s vaccine experts will be tasked with considering whether to extend booster jabs to all over-18s.
Here are the eight key points from Saturday's announcement
1. Face masks will be compulsory in shops and on public transport in England from next week.
2. Anyone entering the UK from any destination abroad will have to take a PCR test two days after arrival and self-isolate until they have a negative result.
3. Anyone who has been in close contact with a case of Omicron will have to self-isolate for 10 days, regardless of their vaccination status.
4. The new measures will be reviewed within three weeks.
5. The two cases discovered in the UK were part of the same outbreak and are in Essex and Nottingham.
6. Omicron can be spread between people who are double vaccinated.
7. The variant has an â€œextensiveâ€ mutation which means in may â€œat least in partâ€ reduce the protection of the vaccine over time, the PM said.
8. Omicron â€œreally changes the risk/benefit calculationsâ€ for the Joint Committee on Vaccination and Immunisation regarding recommending vaccines to younger children, meaning children under 12 could soon be eligible.