Millions more adults and children will "inevitably" be offered a third shot of a COVID vaccine as part of efforts to limit the spread of the Omicron variant.
Experts on the Joint Committee on Vaccination and Immunisation (JCVI) are due to give advice on expanding the booster programme and cutting the gap before a third dose as early as today.
The group are also considering whether second doses should be offered to 12 to 15-year-olds, who currently are only entitled to one.
It comes as nine cases of the new Omicron variant have now been found in the UK - six in Scotland and three in England.
Professor Anthony Harnden, deputy chair of the JCVI, said it was important to ensure any expansion of the vaccine programme was "done in a sensible order" to protect vulnerable groups.
Extending the age range for boosters and reducing the gap between second and third doses was "a sensible strategy", he added.
He told the BBC: "Inevitably, everybody will be offered a booster.
"But what we want to do is make sure that it's done in a sensible order so that those that are most vulnerable for this infection can get boosted and their natural immunity levels can go up."
Health minister Edward Argar told Sky News the government was expecting to receive the JCVI's advice - on whether to expand the booster jabs programme and on cutting the gap between second and third doses - "in the coming hours".
"We'll wait for their advice - that's what they're there for - and we will look very carefully at the advice they give us, hopefully in the next few hours," he said.
The expansion of the vaccination programme is just one part of a host of measures aimed at preventing the spread of the Omicron variant.
Pupils and teachers in year 7 and above are now being "strongly advised" to wear masks in communal areas outside classrooms in England.
Masks become compulsory again on public transport and in shops, banks, post offices, and hairdressers from 4am on Tuesday, when isolation rules will return for international arrivals until they receive a negative PCR test on or before day two.
Mr Argar told Sky News the discovery of the Omicron variant was "concerning" but stressed it was "still early days".
"We've taken swift precautionary steps, we don't fully understand this new variant at all yet," he said.
"It's got 50 mutations roughly, of which 32 are in the spike protein - the bit that connects the key to our cells and infects us.
"So that is concerning, but it's still early days in terms of understanding how it actually reacts in terms of transmissibility, in terms of how dangerous it is.
"Therefore, let's let the scientists do their work but we've taken some proportionate, measured steps to help slow down the spread and the seeding of that variant while they do their work.
"Hopefully it won't be a huge concern but we don't know. We've got to be precautionary in this."
It is hoped the new measures will buy time for scientists to gain a greater understanding of Omicron as ministers put the NHS on notice to deliver many more vaccines every day.
Dr Jenny Harries, 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.
Close contacts of positive Omicron cases have already been ordered to isolate for 10 days even if they are vaccinated under emergency measures announced over the weekend.
Prime Minister Boris Johnson has pledged that the added restrictions will be reviewed in three weeks' time.
However, some experts have suggested that the additional face masks requirement do not go far enough as it excludes hospitality.
Dr Chaand Nagpaul, chair of the British Medical Association, told Sky News that it "doesn't make sense" to ask for masks in shops and on public transport as the virus can spread in "any crowded indoor setting".
The variant has prompted the UK to place 10 African countries on the red list - South Africa, Namibia, Lesotho, Botswana, Eswatini, Zimbabwe, Angola, Mozambique, Malawi and Zambia.
Labour deputy leader Angela Rayner welcomed the government's "swift action" in the face of the Omicron variant.
But she told Sky News the government should "go further" with testing of people before they enter the UK, not just on day two after their arrival, as well as greater action on providing sick pay for those required to self-isolate, increased ventilation in schools and people being encouraged to work from home where possible.
Ms Rayner also called for face coverings to be worn in hospitality venues, such as pubs, as well as on public transport and in shops.
"It's so important that people wear masks when they're indoors, in arenas where they're meeting people and... mixing in large numbers," she said.
"There's no distinction between a pub, sitting in a pub, or sitting on a train, or sitting in a hospital. It's still a venue that's indoors and we should be taking the necessary measures to protect people around us."