Work-from-home guidance will return, vaccine passports will become mandatory in nightclubs and large venues and mask rules will be extended to combat the Omicron variant as Boris Johnson announced a move to "Plan B" to tackle coronavirus.

The British Prime Minister warned it is clear the new strain is "growing much faster" than Delta and cases of Omicron could be doubling every two or three days as he strengthened England's rules to slow the spread of Covid-19.

It comes as British Health Secretary Sajid Javid warned Omicron cases across the UK could exceed one million by the end of this month on the current trajectory.

Guidance to work from home where possible will return from Monday, Mr Johnson said, and mandatory mask wearing will be extended to public places like cinemas and theatres from Friday.

"There will of course be exemptions where it is not practical, such as when eating, drinking, exercising or singing."

The NHS Covid pass will be mandatory in nightclubs and other venues where large crowds gather from "a week's time", the UK Prime Minister told a Downing Street press conference, as he described the measures as "proportionate and responsible".

He said: "We will also make the NHS Covid pass mandatory for entry into nightclubs and venues where large crowds gather, including 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.

"The NHS Covid pass can still be obtained with two doses but we will keep this under review as the boosters roll out.

"And having taken clinical advice since the emergence of Omicron, a negative lateral flow test will also be sufficient."

Mr Johnson said daily testing would be introduced instead of isolation for those who come into contact with infected people.

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He said: "The single biggest thing that every one of us can do is to get our jabs and, crucially, to get that booster as soon as our turn arrives."

The British Prime Minister 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."

In the House of Commons, Mr Javid said there are 568 cases of Omicron confirmed across the UK, but that the true figure is estimated to be "probably closer to 10,000".

He said Omicron cases could exceed one million by the end of this month on the current trajectory.

Mr Javid said: "We're confident that Omicron is significantly more transmissible than Delta. There is still a lot to learn, but some important data has emerged recently.

"It is far more transmissible than the Delta variant."

Delta cases had doubled in around seven days but for Omicron "based on the latest data from here and around the world, our latest analysis is that it's been around, it's between 2.5 and three days.

"This has made the virus an even more formidable foe."

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 300% 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 there 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."

Prof Whitty said: "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."

Last month, ministers at Stormont issued similar working from home advice for people in northern Ireland, while many of the measures announced today are also in effect in Scotland.

Scotland's First Minister Nicola Sturgeon said the Omicron variant's increased transmissibility created "big challenges for the NHS and economy".

She tweeted: "Re Uk gov announcement of Covid Plan B today, all these protections are already in place in Scotland and have helped us get Delta cases down.

"Tough question we all face in period ahead is whether these protections will be strong enough against a rapidly spreading Omicron variant."