New coronavirus restrictions are to be imposed under the government's 'Plan B' in response to rising cases and the spread of the Omicron variant.

At this evening's Downing Street briefing, the Prime Minister announced new guidance to work from home, the extension of face mask rules and the introduction of domestic vaccine passports for events and large venues.

Speaking from the same lecturn where Downing Street officials had been seen laughing in a video, Boris Johnson warned: "Omicron is growing much faster than the previous Delta variant.

"There's been 568 cases across every region of the UK and the true number is certain to be much higher.

"Most worryingly, the doubling time of Omicron could currently be between two and three days."

He added: "It's now the proportionate and responsible thing to move to Plan B in England."

The guidance to work from home if possible would be reintroduced from Monday, he said.

Wearing face masks indoors in public spaces will be mandatory, including in theatres and cinemas, but not when it is not practical, such as eating, exercising or singing.

"Covid vaccine passports will be mandatory for nightclubs, unseated indoors venues with more than 500 people, unseated outdoor venues with more than 4,000 people and any place with more than 10,000 people," the PM added.

"As Omicron spreads in the community we'll produce daily tests for contacts instead of isolation, keeping people safe while minimising disruption to daily lives.

"But the single biggest thing every one of us can do is to get our jabs and get our booster. One year to the day since our first vaccination we've opened up boosters to those over 40. Our heroic NHS staff have already done almost 21 million boosters.

"Let's do everything we can do to protect ourselves and loved ones this winter, and to reduce the pressure on our NHS.

'It's ok to keep going with Christmas parties but obviously everybody should exercise due caution.'

Quizzed on whether Christmas parties and school nativities can still go ahead, Mr Johnson said there was no need to cancel.

"We do not want kids to be taken out of school," he said. "We do not want nativity plays to be cancelled.

"It's ok to keep going with Christmas parties but obviously everybody should exercise due caution.

"Wash your hands, get a test before you go. Give everyone at the party confidence they will be meeting someone who is not contagious."

The PM said the restrictions would be reviewed no later than early January and possibly before then.

Earlier, Downing Street sources insisted “no decisions have been made” amid widespread speculation that further measures could be imminent.

The Cabinet’s Covid-19 operations committee met this afternoon to consider the next steps.

One reason for the rapid shift towards Plan B could be figures from the Scientific Advisory Group for Emergencies (Sage) panel which suggested at least 1,000 people a day could need hospital treatment for Omicron by the end of the year without restrictions.

Leaked minutes of a Sage meeting held yesterday said the government should “urgently” consider the need for measures to reduce transmission of the virus and protect the NHS from “unsustainable pressure”, the BBC reported.

The peak of the Omicron wave is “highly likely to be higher” than 1,000 to 2,000 variant-related admissions per day without new rules, the document said.

But the timing of the announcement has been viewed with 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.

At Prime Minister’s Questions, senior Tory William Wragg challenged Mr Johnson about reports that a Cabinet meeting and press conference were planned “to initiate Covid winter Plan B”.

The chairman of the Commons Public Administration and Constitutional Affairs Committee told the Prime Minister: “Very few will be convinced by this diversionary tactic.”

The Prime Minister told him: “No decisions will be taken without consulting the Cabinet.”

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

Kent County Council has confirmed it is working with the UK Health Security Agency to make sure confirmed and highly probable cases of Omicron in Kent and Medway are found and isolated as quickly as possible.

The government had so far insisted it was not time to activate its Plan B – the restrictions that would be brought in to prevent the NHS being overwhelmed this winter.

But Omicron may have changed those calculations in Downing Street, with Mr Johnson telling the Cabinet yesterday that “early indications were that it was more transmissible” than the Delta strain.

Restrictions 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.