Millions more people could be offered booster jabs as advisers consider extending them to all adults under emergency measures to limit the spread of the omicron variant.
A decision from the Joint Committee on Vaccination and Immunisation (JCVI) on expanding the programme and cutting the gap before a third dose could come as early as Monday.
Teachers and pupils in Year 7 and above are now being “strongly advised” to wear masks in communal areas outside classrooms in England.
They will become compulsory again on public transport and in shops from Tuesday, when isolation rules will return for international arrivals until they receive a negative PCR test.
Close contacts of positive omicron cases were being ordered to isolate for 10 days even if they have been vaccinated under emergency measures announced over the weekend.
Britain will convene an urgent meeting of G7 health ministers on Monday to discuss the variant first detected in South Africa amid concerns it could spread rapidly and partially evade existing jabs.
A third omicron case was detected in the UK on Sunday, in a person with travel links to southern Africa who visited Westminster before leaving the country. It was thought the individual visited the wider borough and not Parliament.
The first two infections were identified in Nottingham and Essex, where officials were ordering PCR tests for customers of a KFC in Brentwood as far back as November 19.
Dr Jenny Harries, the chief executive of the UK Health Security Agency, acknowledged it was “very likely” that further cases of Omicron would be discovered in the coming days.
But Health Secretary Sajid Javid told families they should plan for a “great” Christmas “as normal” and insisted it was “nowhere near” time to reintroduce social distancing rules and work-from-home guidance.
It was hoped the new measures would buy time for scientists to gain greater understanding of Omicron as ministers put the NHS on notice to deliver many more vaccines every day.
JCVI deputy chairman Professor Anthony Harnden said extending the age range for boosters and reducing the gap between second and third doses was “a sensible strategy”.
He told BBC Radio 4’s Broadcasting House that adults aged 18 to 39 should expect third jabs to be offered to them “earlier than we had previously envisaged”.
The group was also considering whether second doses should be offered to 12- to 15-year-olds.
Whitehall sources were hopeful the decision may come on Monday, but could not rule out Tuesday for the JCVI’s announcement.
Education Secretary Nadhim Zahawi recommended that staff, visitors and pupils in Year 7 and above should wear masks in communal areas in schools, colleges and universities such as corridors, canteens and halls in England from Monday.
Though the reintroduction of masks in England in shops and public transport will bring the nation back closer into line with Scotland, Wales and Northern Ireland, the face coverings are not being required in pubs and restaurants.
Mr Javid said it “would be irresponsible to make guarantees” during the ever-changing pandemic, but he told Sky’s Trevor Phillips on Sunday: “I think people should continue with their plans as normal for Christmas, I think it’s going to be a great Christmas.”
With the Government stopping short of introducing its plan B to tackle Covid-19 this winter, Mr Javid played down any need to reintroduce social distancing rules or work-from-home guidance.
He warned of the “very heavy price” of additional restrictions, adding: “So, if one was to make decisions like that they would have to be done very, very carefully and we’re not there yet, we’re nowhere near that.”
Passengers arriving in the UK from 4am on Tuesday will be required to take a PCR test by the end of their second day from entry and isolate until they receive a negative test, while 10 southern African nations have been added to the red list.
Mr Javid admitted that passengers flying in from southern Africa before 10 nations were added to the red list were not tested on landing and they could have taken public transport to get home.
“I think the speed at which we acted could not have been any faster,” he told The Andrew Marr Show on the BBC, adding that those arrivals had been contacted and told to take tests.
Prime Minister Boris Johnson announced the bulk of the “temporary and precautionary” measures on Saturday, pledging that they would be reviewed in three weeks.