The final question asked whether Cabinet Secretary Simon Case would look at all allegations of parties in Downing Street last year and when he would commit to publishing those findings.

In response, Johnson said Case had "got to get to the bottom of what he thinks is appropriate" and repeated his insistence that he had been told all rules were followed.

He added that the findings would be published as soon as possible.

Emilio Casalicchio from Politico asks whether more could have been done to avoid the current situation, including rolling out boosters faster.

The prime minister denies the vaccine rollout has been "sluggish" and says the UK has administered more booster shots than any other comparable European country. But he says "we now need to go much further and much faster" in light of the Omicron variant.

Prof Chris Whitty admits there have been "bumps in the road" and "things we would have done differently", but says a new variant is not "a failure" and is to be expected.

Sir Patrick Vallance, chief scientific adviser, told the news conference the Plan B measures were needed during Covid's current "bumpy transition" period.

"This virus has mutated a lot quickly," he says. "That is sort what you'd expect to see at this stage...

"What we are on is the road from pandemic to endemic - where this becomes a more sort of regular infection like flu over time," he says.

But Prof Vallance adds: "There are special measures that need to happen now to try to reduce the spread. It doesn't mean this is what happens in perpetuity."

"You've got to act to protect public health when you have got the clear evidence," Boris Johnson says in response to suggestions that the Plan B announcement was timed as a diversion from the Christmas party row which has been raging this week.

Asked by the Telegraph's Ben Riley-Smith about his advice on school nativity plays and Christmas parties, the prime minister says people should not cancel them but should follow the guidance.

What are the new rules? You can read about Plan B here.

The Mirror's Political Editor, Pippa Crerar, asks why the PM has decided to ask Cabinet Secretary Simon Case to investigate a Christmas party on December 18 2020, rather than a party in which she says he gave a speech at in November 2020, which would have been in breach of the social distancing rules at the time.

Boris Johnson shakes his head as Crerar asks her question.

He says: "This is a massive department of state, and there are people working flat out the whole time... they work extremely hard".

"No rules were broken," he says, "people within this building have stayed within the rules."

"There will be proper sanctions" if rules have been broken," he adds.

Beth Rigby of Sky News asks the prime minister another question about the resignation of Allegra Stratton over the No 10 Christmas party video.

Boris Johnson says Stratton was an "outstanding" spokeswoman for the government and COP26, and that he was sorry to see her depart.

But he adds that he has not yet seen her tearful resignation video.

Asked if the resignation was evidence of other people taking responsibility for what happens in Downing Street, Johnson says he "doesn't just take responsibility for things that happens in this building, I take responsibility for everything that happens in this government".

Image source, Getty Images

Boris Johnson is asked by Anushka Asthana of ITV about reports of three more parties that may have broken Covid guidelines and why they are not being investigated.

The prime minister says the Cabinet secretary will conduct an inquiry into what took place in Downing Street on 18 December, but says as for other dates "to the best of my knowledge we have followed the rules throughout".

He says it is clearly important Simon Case is "able to get to the bottom of it".

He says he knows it's "contentious", "difficult" and sometimes "the messages are confusing" but they do the best they can.

Sir Patrick Vallance adds that the measures to curb the spread of the virus "only work if we all do it" and says it has been incredibly important everybody follows it throughout.

Image source, UK Government

Releasing some data on the new variant, Prof Whitty says the number of people in hospital had been going down, but the figures have now stabilised.

England's chief medical officer adds the Omicron variant would not be expected to have an impact on hospital admissions for about 2-3 weeks.

He says that Covid is "still a big issue", pointing out there still is a daily average of 121 deaths in the UK of people who have been infected.

Image source, UK Government

In the face of the new Omicron variant, Boris Johnson has announced England will move to "Plan B" restrictions to slow the spread of the virus and buy more time to administer boosters.

What will change:

• From Monday, guidance to work from home will be reintroduced. "Go to work if you must, but work from home if you can," the PM said
• Masks will be required in most indoor public venues like cinemas and theatres from Friday
• The NHS Covid pass, proving vaccination or a negative test, will be required in nightclubs and other venues where large crowds gather from a week's time

Johnson described the measures as "proportionate and responsible".

Meanwhile in the Commons, Health Secretary Sajid Javid has said the true number of Omicron cases in the UK is closer to 10,000. There are fewer than 600 confirmed cases.

Image source, Getty Images

The BBC's Political Editor Laura Kuenssberg says the PM is standing at the lectern "exactly where some of your team laughed and joked about Covid rules," in footage that emerged this week.

She asks how the public can believe him when, she says "you had no idea what was going on under your own roof".

Boris Johnson starts his answer by paying tribute to Allegra Stratton who has resigned from her role in No 10 today.

"There can be no excuse for it, and I can totally understand how frustrating it was" to see that video being released, he states, saying she was a "particularly effective spokesperson" for the COP26 climate summit in Glasgow.

He says it has been a "sad day for her" despite it also being "infuriating for people around the country".

The public can see "the vital importance of the medical information we are giving," he says.

The second question from the public comes from Rachel in Essex, who asks about fully vaccinated travellers having to use quarantine hotels.

It has been necessary to have "very tough border measures" to slow the arrival of Omicron in the UK, Boris Johnson responds.

He says that given the way Omicron is now seeded around the world, the government will be looking at the red travel list and the way "we do it".

In the first of the public questions, Dave from Chester asks why vaccinations are not being made mandatory.

Boris Johnson says "it is not the way we do things in this country" and the British people have shown willingness to get the vaccines voluntarily.

He says, however, there is still a group of people who are refusing to accept vaccinations.

"If we can show the vaccines" are holding out against Omicron then there may well have to be discussions over vaccinations in the future, he states.

Image source, UK Government

The Omicron variant has emerged on the back of a continuingly high rate of infections of the Delta variant, Prof Chris Whitty, England's chief medical officer, says.

Although the new variant is not making a big dent in the daily case figures, "the data here is now clear," he says. "Omicron is going up incredibly fast."

He said the aim of the Plan B measures is to "slow things down" - adding Omicron could go from "very small numbers to very large numbers quickly".

Whitty adds that the booster shot campaign will become critical when Omicron "becomes significant and ultimately dominant".

Why is three-day doubling worrying?published at 18:24 Greenwich Mean Time 8 December 2021

18:24 GMT 8 December 2021

Robert Cuffe
BBC head of statistics

Omicron accounts for a small percentage of cases in the UK. So why the rush to bring in Plan B?

Three-day doubling would take you from 1,000 cases a day to around 100,000 cases a day in one month.

After four weeks, you might only be seeing 50,000 cases a day.

But the next three days, one more doubling, would take it to 100,000. And the three days after that would double it again.

So when the pressure arrives, it arrives very quickly.

Even if all those infections were mild, that’s a lot of people isolating and absent from work or school. Add the potential for sickness to that, even at low rates, and the picture gets worse.

Omicron’s spread, once it’s established in the UK, could be slower or faster than these early estimates. It’s still too early to know.

But this is a reminder of why there’s so much urgent work going on right now to understand Omicron and so much caution.

As we learn more about the Omicron variant, the prime minister says, the government will be guided by four key factors:

• the efficacy of vaccines
• the severity of Omicron
• the speed of its spread
• the rate of hospitalisations

"We must be humble in the face of this virus," Boris Johnson adds.

He says as soon as it becomes clear that boosters are capable of holding the Omicron variant "then we will be able to move forward as before".

He calls on everyone to play their part, before handing over to Prof Chris Whitty.

Iain Watson
Political correspondent

There had been some speculation that proof of vaccination alone would allow entry to nightclubs and large venues in England.

Some ministers and many Conservative backbenchers are opposed to these "vaccine passports".

So the option of demonstrating a negative test may be a sop to some of them - but it has also been Labour's position which should ensure the measures go through if put to a vote (there is an expectation that parliament will get a vote on Plan B next week).

Image source, Getty Images

As Omicron spreads in the community, daily tests will be introduced instead of isolation to "minimise the disruption to daily life" the prime minister says.

"But the single biggest thing that everyone of us can do is to get our jabs and crucially to get that booster as soon as our turn arrives," he says.

He says NHS staff and volunteers have already done almost 21 million boosters but says "we need to go further and faster still" because scientists are confident immune response will be stronger with a booster.

Image source, UK Government

From Friday the legal requirement to wear a face mask will be extended in England to most indoor public venues, such as cinemas and theatres - although there will be exemptions for things such as eating, the PM says.

Covid passes will become mandatory for nightclubs and venues where large crowds gather in England.

This will include unseated indoor venues with more than 500 people, unseated outdoor venues with more than 5,000 people and any venue with more than 10,000 people.

He says two doses will be sufficient for a Covid pass, but this will be kept under review as boosters are rolled out.

A negative lateral flow will also be acceptable.

Among the measures being introduced, the prime minister says, are:

• guidance to work from home will be reintroduced for those who can do so from Monday
• from this Friday, mandatory masks will be extended to most public venues, including theatres and cinemas
• from next week, the NHS Covid pass will be mandatory for entry into nightclubs and venues where larger crowds gather

Health Secretary Sajid Javid is also updating MPs in the House of Commons on Covid at the same time.

He says the UK is “working night and day” to understand more about the Omicron variant.

He says Omicron is “significantly more” transmissible than the Delta variant and analysis suggests it is taking 2.5 to 3 days for cases of the variant to double.

“By the end of this month, infections could exceed one million,” he says analysis by the UK Health Security Agency suggests.