Boris Johnson confirmed that “Plan B” of the government’s coronavirus strategy will be enforced in England.

As part of the new restrictions, the Prime Minister said that people should work from home 'if they can' as of Monday.

Other measures will include vaccine passports at nightclubs and venues with more than 500 people and face coverings would be “further extended” to “most public indoor venues” including theatres and cinemas. Full details of which venues would be impacted are yet to be confirmed.

Boris Johnson told a Downing Street press conference: “We can’t yet assume that Omicron is less severe than previous variants.

“So while the picture may get better, and I sincerely hope that it will, we know that the remorseless logic of exponential growth could lead to a big rise in hospitalisations and therefore, sadly, in deaths.

“That’s why it is now the proportionate and the responsible thing to move to Plan B in England while continuing to work closely with our colleagues in the devolved administrations, so we slow the spread of the virus, buy ourselves the time to get yet more boosters into arms, especially in the older and more vulnerable people.”

Vaccine passports will be mandatory for entry into nightclubs and venues where large crowds gather.

This includes unseated indoor venues with more than 500 people, unseated outdoor venues with more than 4,000 people and any venue with more than 10,000 people.

But Mr Johnson said a negative lateral flow test will also be sufficient.

He added: “As we set out in Plan B, we will give businesses a week’s notice, so this will come into force in a week’s time, helping to keep these events and venues open at full capacity while giving everyone who attends them confidence that those around them have done the responsible thing to minimise risk to others.”

Rules around increased use of facemasks will come into place as of Friday, December 10 and will be required in most indoor venues like cinemas and theatres from Friday.

Daily testing will be introduced instead of isolation for those who come into contact with infected people, it was also announced.

Mr Johnson said the Government would "constantly monitor the data" but called on everyone to "play your part" and get a booster vaccine when available.

He added: "As soon as it becomes clear that the boosters are capable of holding this Omicron variant and we have boosted enough people to do that job of keeping Omicron in equilibrium, then we will be able to move forward as before."

Alleviating fears of further restrictions down the line, the Prime Minister said Plan B measures will not amount to a lockdown and Christmas nativity plays and parties should still go ahead.

"On Christmas, the best way to ensure we have a Christmas as close to normal as possible is to get on with Plan B - irritating though it may be, it is not a lockdown, it is Plan B, it is what we set out a while back - and to get your booster and to get your jab," he said.

Asked about whether festive parties and nativity plays should be cancelled, he replied: "No, in my view they should not.

Adding: "We think that it is OK currently, on what we can see, to keep going with Christmas parties, but obviously everybody should exercise due caution, have ventilation, wash your hands, get a test before you go .

"It is a sensible thing to do to give everybody else at the party the confidence that they are going to be meeting someone who is not contagious."

He said his aim was for restrictions to be in place "no later than early January and possibly before, if we start to get some of that really granular information but we need to see the data and work on it pretty hard".

England’s chief medical officer Professor Chris Whitty said there was an “incredibly steep” increase in cases in South Africa, where Omicron was first detected, and “we are now seeing this translate into increases in hospitalisations”.

He said there was some data suggesting “around about a 300pc increase in hospitalisations over the last week”.

Prof Whitty told a Downing Street press conference that the data in the UK was clear that Omicron cases here were also going up “incredibly fast now”, with a doubling rate between two and three days.

“That is an extraordinarily fast rate and you, therefore, can get with very small numbers to very large numbers really quite quickly.”

He continued: “At the moment the spread is in younger people who you would not expect to go into hospital. It’s once you start moving up the ages and into vulnerable groups that you will start to see that.”

Despite the new rules, as part of efforts to control the Omicron variant, there was no mention of schools in the Prime Minister’s briefing – which can carry on as normal.

But speaking after the press conference, Kevin Courtney, joint general secretary of the National Education Union, said he was “disappointed”.

“Cases are rising so fast and this is leading to so much disruption of education,” he said.

“More than 200,000 pupils were absent for Covid-related reasons on November 25 and since then infection rates among school-age children have risen further.

“It is clear that current mitigations are not enough to keep at bay, especially in the new context of the fast-spreading Omicron variant.”

Boris Johnson said he believed the high vaccine take-up indicated that the British public would be able to “see the need to take to heart and act upon” the Plan B advice being given.

Adding: “That point has been proved time and time again.

“It has never been more conspicuous in the way the public has responded to the vaccine rollout and the way they have done the responsible thing.

“I take my encouragement from the way the public have dealt with it themselves.”