ITV News Health Editor Emily Morgan digs into the detail of the new Plan B Covid rules announced for England
People in England should return to working from home, the prime minister has announced, and anyone wanting to enter large venues such as nightclubs will need to prove their Covid status.
The rule on face mask wearing will also be extended to almost all indoor locations from Friday, Boris Johnson told a Downing Street press conference.
The new work from home guidance will come into force from Monday and Covid status will be required to enter large venues from next Wednesday - vaccine passports or proof of a negative test will provide Covid status.
How much will boosters protect us against the new variant?
Mr Johnson announced he was enacting his coronavirus Plan B in response to the new Covid-19 variant known as Omicron, which he said is " spreading rapidly all around the world".
He said he'd been forced to act due to concerns that a rise in cases "could lead to big rise in hospitalisations and therefore sadly deaths".
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 the 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."
England's chief scientific adviser Patrick Vallance said it would be a good idea to "reduce social contacts" in a bid to reduce the spread of the variant.
Health Secretary Sajid Javid said it is "far more transmissible than the Delta variant".
Updating MPs on the new restrictions, he said the new variant "has made the virus an even more formidable foe."
ITV News Business and Economics Editor Joel Hills reports on how businesses are reacting to the new Covid measures
The prime minister urged people to get their booster jabs, which he said would be the best way for people to protect themselves against the new variant.
He suggested the new restrictions will be removed if scientists ascertain that boosters sufficiently protect against the Omicron variant.
"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."
Watch Boris Johnson's press conference in full:
On the face mask rule, the PM said it would apply in places such as theatres and cinemas but said there "will of course be exemptions where it is not practical, such as when eating, drinking, exercising or singing".
Explaining the new Covid status requirement, Mr Johnson said it will be required in 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".
A negative lateral flow test will be sufficient, as will having protection from two vaccine doses.
In a bid to keep the economy running through the increased restrictions the PM said daily testing would be available instead of isolation for those who come into contact with infected people.
The new measures come on top of a rule enforcing mask wearing in shops and public transport, while international arrivals are subject to strict testing requirements.
MPs have questioned whether the measures are being used as a distraction from accusations that members of the prime minister's staff broke Covid rules last year with a Christmas party.
Mr Johnson earlier apologised after ITV News revealed a video showing staff members, including his former press secretary Allegra Stratton, laughing about the alleged party.
Ms Stratton resigned after the PM said he was "sickened" by the leaked recording and ordered an investigation to establish whether rules were broken.
Watch No10 staff laugh about a party during lockdown restrictions last year
Conservative MP William Wragg said any plan for new Covid restrictions would be "diversionary tactic" to take the focus away from alleged rule breaking in Number 10.
He said: "Covid passes will not increase uptake of the vaccine but will create a segregated society. Is the prime minister aware that very few will be convinced by this diversionary tactic?"
Mr Johnson told the press conference there was "no excuse" for the video but said Ms Stratton had been a "fine colleague".
He said: "I make no excuses for the frivolity with which the subject was handed in that rehearsal that people saw in that clip," the Prime Minister told a press briefing.
"There can be no excuse for it. I can totally understand how infuriating it was."
Critics of the government have suggested the public will be less inclined to follow new coronavirus rules if they believe they haven't been obeyed by the people making them.
Labour leader Sir Keir Starmer said "even the prime minister must understand the damage he's done to his credibility in enforcing the rules now and in the future".
He added: "The British people put the health of others above themselves and followed the rules. Isn't the prime minister ashamed that his Downing Street couldn't do the same?"