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The government "won't hesitate" to introduce further Covid measures in England as cases of the Omicron variant continue to surge ahead of Christmas, the prime minister has said.
Boris Johnson said ministers were "looking at all kinds of things" and would "rule nothing out".
The PM said the data was being reviewed "hour by hour" but there were still "some things that we need to be clearer about before we decide to go further".
He urged people to exercise caution.
A further 91,743 Covid cases were reported across the UK on Monday, the second highest daily total on record.
The government's scientific advisers say new restrictions may be required very soon - and have suggested reducing the size of groups that can meet and closing venues where there is a high risk of transmission.
Speaking after a two-hour cabinet meeting, Mr Johnson said the arguments for taking action in the face of the spread of Omicron were "very, very finely balanced".
He added: "Unfortunately I must say to people that we will have to reserve the possibility of taking further action to protect the public, to protect public health, to protect our NHS."
Asked about the prospect of further restrictions, Mr Johnson said: "At the moment, what I think we want people to focus on is exercising caution - so ventilation, masks in the appropriate places, all the usual stuff about washing hands, but remember how contagious Omicron really is."
He acknowledged the impact of recent behavioural changes by some people, such as cancelling bookings, was having on businesses in the hospitality sector, saying the economic impacts would be kept "under constant review".
Analysis
By Damian Grammaticas, political correspondent
The PM had been urged to give clarity. The nation was waiting to hear if there would be more restrictions coming - and if so, what and when?
Boris Johnson and his cabinet had spent more than two hours deliberating. But what Mr Johnson told us didn't answer those questions.
On how serious things are, he said there were still "uncertainties" and "we should keep the data under review".
On what possible action might come, he said: "We are looking at all kinds of things."
Could it still happen before Christmas? "We will rule nothing out," he replied.
So this was a decision by the PM not to do more right now, despite the fact he said cases were "surging".
Was that because of doubts around the cabinet table? Or because the PM - as he pointed out - believes people are already adapting their behaviour and he wants more time to see if that's enough to control the spread of Omicron?
Perhaps, but it perpetuates the uncertainty. And, as the scientists have said, delay has consequences in itself, because cases continue to rise.
Shadow health secretary Wes Streeting accused Mr Johnson as having "absolutely nothing new to say, no plan whatsoever".
He told Radio 4's PM programme: "Boris Johnson is paralysed by weakness because of fear of his own backbenchers... no-one wants to bite the bullet and level with the public this side of Christmas."
The current rules for England - known as Plan B - include Covid passes for certain events, face masks in more places and people being urged to work from home if they can.
Other nations of the UK have similar rules, though Scotland has gone further by asking people to limit social contact to three households at a time in the run-up to Christmas, and could go further still in the new year.
Wales has also ordered nightclubs to close from 27 December.