Ministers are targeting a return to half a million UK Covid jabs a day as the waiting time for boosters was cut to three months in a bid to outpace the Omicron variant that scientists believe is already spreading in the community.

Confirmed Omicron cases rose to 11 in England and Scotland on Monday, with scientific advisers braced for hundreds more to be detected in the next week or so.

From Tuesday, masks will be mandatory on public transport including airports and stations and in shops – including hair salons and takeaways but not pubs or restaurants – to slow the spread of Omicron, which is feared to more transmissible with the potential to evade vaccines.

The NHS is set to confirm an expansion of the vaccine programme this week after the government’s advisers said all adults should be offered boosters and made the surprise recommendation of a three- rather than six-month wait after a second dose.

A senior government source told the Guardian ministers were aiming for a “significant acceleration” from the current 2.4m boosters a week to about 3.5m or 500,000 a day – a return to the huge national effort seen in the early days of the vaccination campaign. “That is the early plan but it won’t happen overnight,” they said.

Currently, boosters are restricted to over-40s more than six months since their last jab. The first new cohort of people to be offered boosters is likely to be those over-40s and the clinically vulnerable who are more than three but less than six months past their second jabs. The next tranches will be under-40s in staggered age groups from oldest to youngest. Children aged 12 to 15 will be offered second doses for the first time, and people who are severely immunosuppressed will get boosters in addition to three primary doses.

The biggest unvaccinated group remains under-12s. Dr June Raine, chief executive of the Medicines and Healthcare products Regulatory Agency (MHRA), said this was being “very carefully assessed” and the agency is likely to report before Christmas on the safety of vaccines for children aged 5-11. It would also have to be approved by the Joint Committee on Vaccination and Immunisation (JCVI) before a final decision being taken by government ministers.

As the threat of the Omicron variant prompted the JCVI to ramp up the jabs rollout, Prof Jonathan Van-Tam, the deputy chief medical officer, said people needed to “up their game” in terms of getting boosters.

But he added: “I do not want people to panic at this stage. If vaccine effectiveness is reduced [with Omicron], as seems pretty likely to some extent, the biggest effects are likely to be in preventing infections and hopefully there will be smaller effects on preventing severe disease.”

Prof Wei Shen Lim, chair of the JCVI, said Moderna or Pfizer boosters would push up people’s immune response and cutting the time between doses could help mitigate any drop in vaccine protection before another wave starts.

The planned extension of the booster programme means another 13m under-40s will become eligible for third jabs in the coming weeks. The NHS is currently administering 350,000 daily boosters, or about 2.4m a week, and would have to increase to 500,000 a day to hit 3.5m a week.

An NHS source said there were 2,2oo vaccination centres in operation which means that, on average, every site would have to vaccinate 70 more Britons a day to hit 3.5m boosters a week. Sources said it was easier to expand opening hours than open new centres, raising the prospect of more “round-the-clock” provision.

Boris Johnson published details on Monday of new regulations to make mask wearing compulsory on transport and in shops from 4am on Tuesday. This includes hair salons, taxis, vets, takeaway shops, driving instruction vehicles, banks and post offices, though hospitality settings are exempt.

All travellers into the UK will also have to take a PCR test on the second day after arrival and isolate until they receive a negative test. In addition, all contacts of people suspected to have Covid with the Omicron variant must isolate, even if they are double vaccinated or under 18.

Ahead of a vote on the measures in the House of Commons on Tuesday, Johnson said: “The measures taking effect today are proportionate and responsible, and will buy us time in the face of this new variant … our vaccines and boosters remain our best line of defence.”

The measures will be reviewed in three weeks, but some backbench Tories are already concerned about possible extensions. Mark Harper, one of key MPs behind the Covid Recovery Group, warned measures should not be extended during parliament’s recess just before Christmas. “Trust between the backbenchers and ministers isn’t brilliant at the moment. The government would be wise not to make that worse,” he said.

Sajid Javid, the health secretary, told parliament that if Omicron was no more dangerous than the dominant Delta variant “then we will not keep measures in place a day longer than necessary”.