Boris Johnson has introduced new measures to tackle the Omicron variant of coronavirus.
New restrictions on face coverings came into force last week, while rules around international travel and testing were also tightened.
It is now compulsory to wear a mask in England in shops and settings such as banks, post offices, hairdressers, and public transport.
All travellers arriving into the UK must also take a PCR test and self-isolate until they receive a negative result.
The UK’s travel red list - the countries that require a 10-day period of isolation in a hotel upon arrival back in the UK, has also been expanded to include 10 African nations: Angola, Botswana, Eswatini, Lesotho, Malawi, Mozambique, Namibia, South Africa, Zambia and Zimbabwe.
All contacts of any suspected Omicron case must also self-isolate when told to do so by NHS Test and Trace, regardless of their age or vaccination status.
The measures in place have been described by the government as “temporary and precautionary”.
As he announced the new restrictions, the prime minister said they would be reviewed after three weeks.
The measures came into force on Tuesday, November 30, which means a review is expected by Tuesday, December 21.
Reports suggest that the laws around wearing masks in shops and on public transport will stay in place until January, according to the Mirror.
A member of the Scientific Advisory Group for Emergencies (SAGE) said restrictions could still be needed in January to give scientists more time to understand the impact Omicron may have on the pandemic.
Professor Sir David Spiegelhalter, of the University of Cambridge, said the new variant “may actually be milder but we haven't got enough data yet to be able to say”.
Speaking to Sky News the statistician added: “Just like last year with the Alpha variant, this is a terrible time for a variant to come along.
“The next two weeks are probably some of the highest-risk periods with a lot of indoor socialising that we know is extremely high risk."
The government also announced it would be further tightening restrictions for international arrivals in the UK over the weekend.
From Tuesday, December 7, all travellers arriving in England will be required to take a Covid-19 pre-departure test in an attempt to limit the spread of the Omicron variant.
The move has been welcomed by scientists as a way of buying time to learn more about the new variant, but travel experts have said the rules around testing are a “huge blow” for the travel industry.
On a visit to Merseyside today, the prime minister defended the new measures.
Mr Johnson told reporters: “What we’re doing is responding to the pandemic.
“We were the first country in the world to take decisive measures to tackle Omicron. We put about 10 countries automatically, immediately, on to the red list and we said that anybody coming from any country in the world would have to quarantine for a couple of days.
“We’re now going further and toughening those measures up as we see the spread of Omicron around the world.”
He added that the government does not believe it needs to change the “overall guidance and advice we’re giving about Omicron in this country”.
"We’re still waiting to see exactly how dangerous it is, what sort of effect it has in terms of deaths and hospitalisations," he said.
Meanwhile, in the United States, President Joe Biden’s chief medical adviser Dr Anthony Fauci said early indications suggest Omicron may be less dangerous than Delta.
“Thus far, it does not look like there’s a great degree of severity to it,” Dr Fauci said.
“But we have really got to be careful before we make any determinations that it is less severe or it really doesn’t cause any severe illness, comparable to Delta.”
Even if Omicron proves less dangerous than Delta, it remains problematic, World Health Organisation epidemiologist Dr Maria Van Kerkhove said.
“Even if we have a large number of cases that are mild, some of those individuals will need hospitalisations,” she said.
“They will need to go into ICU and some people will die. We don’t want to see that happen on top of an already difficult situation with Delta circulating globally.”
A total of 336 cases of Omicron – also known as B.1.1.529 – have been confirmed in the UK so far.
Sajid Javid is expected to make a statement on the coronavirus pandemic to MPs in the House of Commons on Monday afternoon.
