Face coverings have become mandatory again on public transport and shops in England in an effort to slow the spread of the Omicron coronavirus variant.
People arriving in the UK from abroad will now also have to take a PCR test and self-isolate until they receive a negative result.
It comes after 11 cases of the new variant were detected in the UK.
Health Secretary Sajid Javid said it is expected cases will continue to rise in the coming days.
On Monday the government announced a major expansion of the booster jab rollout as part of efforts to help stop a potential wave of infections. They are to be offered to all over-18s in the UK, while children aged 12 to 15 will be invited for a second jab.
NHS England said it will “shortly set out how staff will expand the booster programme – this will include how booster jabs will be given in priority order… while also increasing capacity to vaccinate millions more people in a shorter space of time”.
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“Our experience of fighting this virus has shown us it’s best to act decisively and swiftly when we see a potential threat. Which is why we’re building our defences and putting these measures in place without delay.”
Early evidence suggests Omicron – first found in southern Africa – has a higher re-infection risk. But scientists say it will take about three weeks before it is known how the variant impacts on the effectiveness of vaccines.
The changes to the rules began at 04:00 GMT.
Another change, which has already come into effect, requires all contacts of suspected Omicron cases to self-isolate for 10 days, even if they are fully vaccinated.
The government says it will review the changes again in three weeks’ time and the health secretary told MPs they would be given a chance to debate and vote on the rule changes on masks and self-isolation.
He said: “If it emerges that this variant is no more dangerous than the Delta variant, then we won’t keep measures in place for a day longer than necessary.
“Our experience of fighting this virus has shown us it’s best to act decisively and swiftly when we see a potential threat. Which is why we’re building our defences and putting these measures in place without delay.”
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Mr Javid added: “The reason we’ve taken measures is to protect the progress we have made, and if we can do that then we can look forward to a fine Christmas.”
The new rules on face coverings bring England in line with Scotland, Wales and Northern Ireland, where they are already mandatory on public transport and for many indoor areas.
Newly-published guidance says they should be worn in locations including shops, shopping centres, posts offices, banks, hairdressers and food takeaways, as well as on public transport.
Although the change does not require people in England to wear coverings in pubs and restaurants, they are required in hospitality venues elsewhere in the UK in some circumstances.
Speaking in the Commons, Labour’s shadow health minister Dr Rosena Allin-Khan said the government should not have stopped asking the public in England to wear masks and that the rules should now be extended to include hospitality.
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Meanwhile, the first ministers of Scotland and Wales have called for the PCR test requirement to be changed so that everyone arriving in the UK would have to isolate for eight days.
The latest cases of the Omicron variant were detected in the Camden and Wandsworth areas of London, the UK Health Security Agency said. Like the other three cases in England, they have been linked to travel in southern Africa.
First Minister Nicola Sturgeon said some of the six cases identified in Scotland had no travel history. This suggested a degree of community transmission, she said, but there was currently no suggestion this was widespread.
The average number of daily confirmed Covid cases in the UK began rising again in early November. A further 42,583 cases were reported on Monday.