Boris Johnson's top team will meet this afternoon to review the latest Covid data and weigh up whether tougher restrictions will be needed over Christmas.
Chief Medical Officer Chris Whitty and Chief Scientific Adviser Sir Patrick Vallance will brief the Cabinet at 2pm on the spread of the Omicron variant.
Confirmed cases of the faster-spreading strain have risen by more than 12,000 in the UK – and London’s cases alone topped 10,000, according to the latest data on Sunday.
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Advice from the Scientific Advisory Group for Emergencies (Sage), published at the weekend, warned there are likely already hundreds of thousands of new Omicron infections every day in England and that hospital admissions with the variant in the UK are “probably around one tenth of the true number” due to a lag in reporting.
Asked if Cabinet would be signing off on tougher restrictions this afternoon, the prime minister's spokesman said: "We need to strike the right balance between protecting lives and livelihoods. That's what we are focused on.
"This is a fast moving situation. There are significant gaps in the data that we have relating to this variant and so we are working to get more clarity on what impact it has particularly on things like severe illness, hospitalisations and deaths.
"But in the meantime, we are not waiting we are encouraging people to get boosted and have introduced measures to slow the spread of this variant through our plan B."
He added: "We would want to avoid any position which might put our NHS at risk, that is very much at the forefront of our mind when looking at the data."
The spokesman said the public would be updated at "the earliest possible opportunity" if a decision was taken to introduce tougher measures.
It came as deputy prime minister Dominic Raab said he could not make any “hard, fast guarantees” that more restrictions will not be needed ahead of Christmas Day.
He said: "We will have a much better Christmas than last year because of the vaccination level, both the overall vaccination level but particularly the impact of the booster campaign, and we saw on Saturday, just in one day, 900,000 people get their booster.
“I do think of course, you have heard it from the Health Secretary over the weekend, I’d echo that, people will need to be careful and cautious.
“I do think that again, subject to the data, and it’s always under review, we’re in a better position to enjoy Christmas with loved ones this year.”
