Professor Chris Whitty has warned ministers to brace for a “significant increase in hospitalisations” from Omicron, as Downing Street insisted no further coronavirus restrictions are being planned.

England’s chief medical officer also told a virtual Cabinet meeting on Tuesday that it is “too early to say how severe” the strain is after early suggestions from South Africa that it could be relatively mild.

Boris Johnson also warned his ministers that he believes a “huge spike” of cases of the variant will hit the nation, as he pressures Tory rebels to back his new Plan B restrictions during a Commons vote.

Downing Street did not reject a suggestion that new Omicron cases could hit a million per day next week based on a “valid” estimate from the UK Health Security Agency that daily infections are currently around 200,000.

Deputy Prime Minister Dominic Raab insisted Plan B restrictions including mandatory mask-wearing and the use of Covid health certificates for large venues will be sufficient over Christmas, meaning families can “spend it with loved ones”.

But Scottish First Minister Nicola Sturgeon urged people across Scotland to limit their mixing by socialising with only up to two other households indoors either side of Christmas.

No 10 stopped short of giving the same advice when asked, but the Prime Minister’s official spokesman said people should “get tested beforehand, to access testing, to use ventilation, to use good hand (and) respiratory hygiene, wash your hands, and that is the advice that we give to the public at this time”.

The downbeat assessment from Prof Whitty delivered to the first Cabinet meeting not to take place in person for months will raise concerns that additional measures may be needed in England in the new year.

He told ministers he can not yet determine whether cases are plateauing in South Africa, where the strain was first identified, and said there is “no reliable evidence” from the nation’s scientists of a peak in case rates.

“He added that it remained too early to say how severe the Omicron variant was but that we can expect a significant increase in hospitalisations as cases increase,” according to No 10’s account of the meeting.

Earlier, South African Medical Association chairwoman Dr Angelique Coetzee told MPs on the Science and Technology Committee that most cases of Omicron have been mild.

Downing Street gave its backing to the UK Health Security Agency estimate that there could already be around 200,000 cases of the rapidly-spreading strain in circulation.

“We think it is a valid figure which is recognising it is an estimate and models have wide ranges,” the Prime Minister’s official spokesman said.

“It does provide an understanding of how quickly Omicron is transmitting, we expect it to become the dominant variant in London today.”

The spokesman did not reject a suggestion that by the middle of next week there could be a million Omicron infections a day, instead saying: “Like I say, cases are increasing significantly.”

However, despite the figures, he insisted there are “no plans” to move beyond Plan B restrictions, which are expected to be backed in Tuesday’s Commons vote with Labour’s support.

“We need to learn more about this variant on things like severity before we decide what – if any – action is required in the future,” he added.

No 10 also insisted there were “robust” measures in place to protect public services.

And that “any discussion or any warning about hospitals or clinical settings having to close as a result of this variant” was incorrect.

Downing Street also insisted there is significant testing capacity after it emerged there are no available slots for PCR tests at walk-in or drive-through sites in some parts of England for people with coronavirus symptoms.

The spokesman insisted ministers are working with Royal Mail and Amazon to expand the number of delivery slots as lateral flow tests became unavailable to be ordered through the Government website for the second day running.

Ministers are pinning their hopes on Plan B and an acceleration of the booster programme to offer all adults in England a third vaccine dose by the end of December to contain Omicron.

I think people can look forward to spending Christmas with loved ones in a way that we couldn’t last year

Mr Raab told Times Radio that the target is “demanding”, but added that it will allow the nation to “go into this Christmas with the ability to spend it with loved ones in a way that was impossible last year”.

But he did not rule out further measures being considered for Christmas or New Year, saying: “These issues are always discussed but we have got Plan B, that’s what we think is required over the Christmas period.”

Pressed if Christmas will be safe, Mr Raab replied: “Yes, I think it is. I want to give that reassurance. I think people can look forward to spending Christmas with loved ones in a way that we couldn’t last year.”

The assurance came as more than 70 backbench Tories threatened to defy the whips and oppose the Government’s Plan B for England, in what would be the biggest rebellion against the Prime Minister since he entered No 10.

Whilst we all support the booster campaign, and I urge all employees to get their booster, the unintended consequence has been a really rather chilling effect on demand in a lot of sectors in our economy

Confederation of British Industry (CBI) director-general Tony Danker said warnings from the Government are creating a “chilling effect” on many sectors of the economy.

He told BBC Radio 4’s Today programme: “It seems to us that, whilst we have measures to keep the economy open, we have messages that have ended up closing much of it down.

“People should be worried enough to go and get a booster urgently, but not so worried to stop going to shops, restaurants or airports.

“That’s what’s not working, you have a double whammy for businesses in those sectors, hospitality, retail, leisure or travel, where demand is collapsing and there’s no support to recover.”

NHS England said it experienced its busiest Monday for vaccinations since the rollout began, with 418,000 booster jabs delivered.