Mandatory mask-wearing will return to shops and public transport on Tuesday, Health Secretary Sajid Javid said as a third case of the Omicron variant of coronavirus was detected in the UK.

The Cabinet minister told families they should plan for Christmas “as normal” and said it is “nowhere near” time to reintroduce social distancing rules and work-from-home guidance, despite a raft of precautionary measures being reintroduced to tackle the concerning strain.

But later on Sunday, the UK Health Security Agency (UKHSA) said it had identified another case of Omicron in an individual with links to travel in southern Africa who visited Westminster in London before leaving the country.

Dr Jenny Harries, the UKHSA chief executive, acknowledged it is “very likely” that further cases of Omicron, which is feared to spread rapidly and may evade existing vaccines to a degree, will be discovered in the coming days.

Targeted testing is being carried out in locations where the latest positive case visited while likely to be infectious, as well as in Nottingham and Brentwood, Essex, where the first two cases of Omicron were identified.

The detection of the strain classified as a “variant of concern” by Government scientists came ahead of a new testing regimes being introduced under measures to slow its spread.

International arrivals entering the UK from Tuesday morning will have to take a PCR test for Covid-19 and self-isolate until they get a negative result.

All contacts with a suspected case of Omicron will have to isolate for 10 days, regardless of their vaccination status, under the emergency measures announced over the weekend.

Mr Javid said the compulsory use of masks in shops and public transport, but not in pubs and restaurants, will come into force in England on Tuesday, bringing the nation back closer into line with Scotland, Wales and Northern Ireland.

He 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 downplayed there being a need to reintroduce social distancing rules or work-from-home guidance.

“We know now those types of measures do carry a very heavy price, both economically, socially, in terms of non-Covid health outcomes such as impact on mental health,” he told Sky.

“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.”

Mr Javid said he expects to receive new advice “imminently”, within the next couple of days, from the Joint Committee on Vaccination and Immunisation (JCVI) after it was tasked with reviewing whether boosters should be extended to all over-18s.

The group will also consider whether second doses should be offered to 12 to 15-year-olds, and whether the waiting time before a booster jab could be reduced.

Professor Anthony Harnden, the JCVI’s deputy chairman, told BBC Radio 4’s Broadcasting House that extending the age range for boosters and reducing the delay before receiving them is “a sensible strategy” and told under-40s to expect third jabs to be offered to them “earlier than we had previously envisaged”.

Mr Javid said the testing regime for international arrivals will be introduced “as soon as possible”, despite online passenger locator forms stating PCR tests will be required rather than lateral flow tests from 4am on Tuesday.

Scotland, Wales and Northern Ireland said they intend to mirror the border restrictions.

He 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 return home.

“I think the speed at which we acted at could not have been any faster,” he told The Andrew Marr Show on the BBC, adding that those arrivals have been contacted and told to take tests.

Professor Neil Ferguson, the Scientific Advisory Group for Emergencies (Sage) member whose modelling helped instigate the first lockdown, said he expects to see “substantially larger numbers” of Omicron in the UK in the coming days, but he welcomed the new measures as “proportionate” to slow the spread while scientists analyse its properties.

“That’s not to say we can be complacent, if we do see very rapid growth of Omicron – and that’s a big if at the moment, and we have no guarantee we will – but if we do, then, undoubtedly, I think the Government would be wise to keep all options on the table in terms of how to respond to that,” he told BBC Radio 4’s The World This Weekend programme.

Yvette Cooper, the Labour MP who chairs the Commons Home Affairs Committee, urged the Government to “immediately” reinstate pre-travel tests because the new approach “leaves far too many gaps”, with potentially infected passengers able to travel home on public transport.

Boris Johnson announced the “temporary and precautionary” measures on Saturday, pledging they will be reviewed in three weeks, but he did not announce the Plan B measures that include work-from-home guidance and vaccine passports for England.

Government advisers on Sage said during a meeting on October 14 that home working is likely to have the greatest individual impact on transmission out of the measures in Plan B, which they said would have the greatest impact if introduced in unison.

To further slow the arrival of cases, ministers said Malawi, Mozambique, Zambia and Angola face travel restrictions from Sunday, when they joined South Africa and five other neighbouring nations on the UK red list.

Mr Javid told Times Radio it is planned that the new regulations will be laid in Parliament on Monday, with MPs expected to be given a vote within 28 days and after they come into force.

A number of backbench Tories may stage a rebellion, but it is thought unlikely Labour would oppose the restrictions, virtually guaranteeing that they will pass.