The Covid update comes hours after the Downing Street Christmas Party scandal led to the resignation of the Prime Minister’s former adviser Allegra Stratton.
Stratton offered her “profound apologies” in an emotional statement after footage emerged of her joking about a Downing Street Christmas party.
Mr Johsnon was joined by England’s Chief Medical Officer Chris Whitty and Chief Scientific adviser Sir Patrick Vallance as he introduced Plan B measures to battle a rise in Omicron cases.
It is understood Health Secretary Sajid Javid will update MPs in a Commons statement at some point on Wednesday evening.
Here are the key points you need to know from the press conference.
Why is Covid Plan B needed?
Boris Johnson confirmed it was time to move to “Plan B” of the Government’s coronavirus strategy, with measures including a return to working from home.
He told a Downing Street press conference: “It has become increasingly clear that Omicron is growing much faster than the previous Delta variant and is spreading rapidly all around the world.”
While 568 cases had been confirmed in the UK “the true number is certain to be much higher”.
“Most worryingly, there is evidence that the doubling time of Omicron could currently be between two and three days.”
New Covid rules introduced in England
Guidance to work from home is to be reintroduced in England as part of measures to halt the spread of the Omicron variant, Boris Johnson has announced.
From this Friday, face masks will be required to most indoor venues including theatres and cinemas for wearing face masks.
The NHS Covid pass is to be made mandatory in England in a week’s time for nightclubs and venues where large crowds gather, Boris Johnson has announced.
These are 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 passes will still work for people who have had just two doses of a COVID vaccine, although this will be kept under review.
Boris Johnson urges people in England to work from home
Boris Johnson told a Downing Street press conference: “We will reintroduce the guidance to work from home.
“Employers should use the rest of the week to discuss working arrangements with their employees but from Monday you should work from home if you can.
“Go to work if you must but work from home if you can.
“I know this will be hard for many people but by reducing your contacts in the workplace, you will help slow transmission.”
When will Plan B come into force in England?
Boris Johnson said businesses will be given a week to make preperations for people to work from home.
From MOnday people will be advised to work from home if they can.
From Friday, face masks will become mandatory in most indoor venues