The Government has brought in their Plan B measures in England to slow the spread of the Omicron variant, which looks increasingly likely to take over from Delta as the dominant strain of coronavirus globally.

But the timing is being viewed in suspicion in Westminster, coming as the Prime Minister was forced to apologise after footage emerged of senior aides joking about a Downing Street Christmas party during last December’s lockdown.

The UK Health Security Agency said 568 cases of Omicron have been confirmed, up 131 since Tuesday’s figure.

A prominent member of the Scientific Advisory Group for Emergencies (Sage) warned that a full UK-wide lockdown to deal with the threat of the Omicron variant cannot be ruled out, although the current threat posed by the strain remains unclear.

Restrictions, which will come into effect on Monday could play a role in slowing the spread of the variant in order to allow more time for the booster jab vaccination campaign to progress.

But what exactly is the Government's Plan B?

Ministers have kept most of their Plan B measures in reserve for if Covid cases rise so high that they place the NHS under unsustainable pressure.

They have already introduced mandatory face masks in shops and on public transport in England and, on November 27, promised to review the measures after three weeks.

The remaining restrictions in Plan B include the introduction of mandatory vaccine passports for larger events and nightclubs.

These passports would prove people are double-jabbed and can attend the event.

The Government has previously said it will seek to give businesses at least one week’s notice before mandatory vaccine certification is enforced.

Mandatory mask wearing will be extended to indoor public venues including cinemas, theatres and places of worship from Friday but will not be required in pubs and restaurants, while the guidance to work from home where possible will return on Monday.

The NHS Covid pass, which can be obtained by having two vaccines or a negative lateral flow test, will be introduced for entry into nightclubs and other large venues from December 15, as Mr Johnson set out the “proportionate and responsible” measures.

But he was forced to insist the public understands the “vital importance” of the measures as he faced questions at the hastily-arranged Downing Street press conference over how they can accept his rules amid anger over allegations staff broke Covid rules in a party on December 18 last year.

Will the plan slow the spread of Omicron?

Professor Neil Ferguson, from Imperial College London, whose data was instrumental to the UK going into lockdown in March 2020, said on Wednesday that “case numbers of Omicron are doubling at least every three days, maybe even every two days at the moment, so it’s accelerating very fast”.

He said lockdowns are a possibility and cannot be ruled out, but working from home guidance could slow the spread.

“There is a rationale, just epidemiologically, to try and slow this down, to buy us more time principally to get boosters into people’s arms, because we do think people who are boosted will have the best level of protection possible, but also to buy us more time to really better characterise the threat,” he said.

He suggested “a kind of Plan B Plus with working from home might slow it down” rather than stopping Omicron, reversing the doubling time to every five or six days.

What does the Scientific Advisory Group for Emergencies (Sage) think?

Sage has said Plan B measures will have the greatest effect if brought in in one go.

Of the individual measures, the scientists advising Government believe working from home will have the biggest impact on slowing the spread of the virus.

The React study from Imperial College London showed working from home reduced the chance of catching Covid-19 during earlier stages of the pandemic.

Analyses of risk by occupation also shows a lower risk for those jobs with higher levels of working from home.

How will England’s Plan B differ from the rest of the UK?

Each of the four nations’ handling of the pandemic is managed by their own leaders.

In Scotland, vaccine passports are already in force and have been since October, with people who are attending nightclubs, indoor events (unseated) with 500 or more people, outdoor events (unseated) with 4,000 or more people and any event with 10,000 or more to show they are double vaccinated before entering.

Since December 6, a negative PCR test taken within 24 hours of entry to a venue or a negative lateral flow test have also been accepted as part of the passes.

Scotland’s Covid passes are called the NHS Scotland COVID Status app.

In Wales, vaccine passports are in force in cinemas, theatres, concert halls as well as nightclubs and large events.

They are also needed for unseated indoor venues with more than 500 people in the audience, outdoor or indoor unseated venues with a capacity over 4,000 and any event with more than 10,000 people.

The passes can be downloaded by people who are double vaccinated or have tested negatively within 48 hours of entering the venue.

Similarly to England, Wales uses the NHS Covid Pass.

Northern Ireland has followed the same rules as Wales, which have been in place since November 29. Enforcement will be applied from December 13.

Residents who can download Covid passes include those who are double vaccinated or have tested negatively within 48 hours of entering the venue.

In Northern Ireland, the pass is called COVIDCert NI Mobile App.