Face coverings have become mandatory again on public transport and shops in England under measures to combat the new Omicron variant of coronavirus.
Masks are already mandatory indoor venues in Wales, Scotland and Northern Ireland.
The return of face coverings in England is just one of several new rules to come in as of 4am on Tuesday.
Travellers returning to the UK must take a PCR test and self-isolate until they receive a negative result and all contacts of suspected Omicron cases must self-isolate, regardless of their age or vaccination status for 10 days.
The UK now has 11 confirmed cases of the variant - which was first detected in South Africa - after the Scottish government announced on Monday it had discovered four cases in Lanarkshire and two in the Greater Glasgow and Clyde area.Health Secretary Sajid Javid said it is expected cases will continue to rise in the coming days.
The Joint Committee on Vaccination and Immunisation (JCVI) is now advising that all adults aged 18 to 39 should be offered a booster dose of the Covid-19 vaccine, in order of descending age groups, to increase their level of protection.
Those aged 40 and over are already eligible for a booster vaccine.
The JCVI said booster doses should be given no sooner than three months after people have had their second dose of an original vaccine – shaving three months off the current six-month wait.
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In further advice, young people aged 12 to 15 should be offered a second dose of the Pfizer/BioNTech Covid-19 vaccine, no sooner than 12 weeks after their first dose.
The NHS said it will shortly set out how staff will expand the booster programme.
A spokesperson said this will include how booster jabs will be given in priority order so that the most vulnerable people are protected first, while also increasing capacity to vaccinate millions more people in a shorter space of time.
From 4am on Sunday, non-UK and non-Irish residents who have been in Malawi, Mozambique, Zambia and Angola in the previous 10 days will be refused entry into England, officials have said.
South Africa, Botswana, Lesotho, Eswatini, Zimbabwe and Namibia were added to the UK’s travel red list on Thursday.
From 4am on Sunday, anyone who arrives in the UK from these countries must quarantine in a government-approved hotel for 10 days at a cost of £2,285 for one adult.
Prime Minister Boris Johnson said: “The measures taking effect today are proportionate and responsible, and will buy us time in the face of this new variant.
“Based on everything we know, our vaccines and boosters remain our best line of defence, so it is more important than ever that people come forward when eligible to get boosted.
“Not only will today’s steps help us slow down the variant’s spread, but they will help us protect each other and the gains we have all worked so hard for.”
The measures in place are described by Number 10 as “temporary and precautionary”, and will be reviewed in three weeks.
England’s deputy chief medical officer Professor Jonathan Van-Tam told a Downing Street briefing on Monday that scientists around the world agree that the Omicron variant is “of increased concern”.
He said there are still uncertainties about how transmissible the variant is and its impact on severity of disease, adding that the “number of mutations present, already on first principle, makes us worry about a possible effect on vaccine effectiveness”.
MPs will debate and vote on new Covid restrictions in England, including face coverings in shops and public transport, on Tuesday.
A number of Tories are deeply unhappy at the prospect of a return of controls – but with Labour backing the measures there is little chance of a government defeat.
Meanwhile, England is the only UK nation where working from home is not encouraged.