Face coverings will become mandatory again in shops and on public transport in England from next week as part of measures to target the new coronavirus variant, Omicron, the PM has said.
PCR tests for everyone entering the UK will be introduced and all contacts of new variant cases will have to self-isolate, even if fully jabbed.
But Boris Johnson said Christmas would be "considerably better" than in 2020.
The measures were "temporary and precautionary", he added.
Mr Johnson announced the restrictions at a Downing Street news conference after it was confirmed that two Omicron cases had been detected in Brentwood, Essex, and Nottingham. Officials said the cases were linked and connected to travel in southern Africa.