Following a special two-hour meeting of the Cabinet the Prime Minister said they were monitoring the data “hour by hour”.
However, he said the arguments for taking action were “very, very finely balanced” – indicating fresh measures were unlikely before Christmas.
“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,” he said.
“We are looking at all kinds of things to keep Omicron under control and we will rule nothing out.
“But 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.”
Ministers were briefed at the virtual meeting on the latest situation by the Government’s chief scientific adviser Sir Patrick Vallance and England’s chief medical officer Professor Chris Whitty.
It comes after the Government’s Scientific Advisory Group for Emergencies (Sage) warned daily hospital admissions could reach 3,000 without further restrictions.
However Mr Johnson said there were still “uncertainties” around the severity of the new strain, as well as the rate of hospital admissions associated with it, and its impact on the effectiveness of the vaccines.
At the same time he said they needed to take account of the economic impact of any new measures – particularly on those sectors which were already struggling due to the pandemic.
“There are still some things that we need to be clearer about before we decide to go further,” he said.
“It is important we act cautiously but it is also important we look after the hospitality industry, the theatres and other parts of our incredible entertainment industry which have suffered and are suffering.”
There was relief among some Conservative MPs who rebelled last week over the Government’s plans for Covid passports that ministers had stopped short of imposing controls in the final days before Christmas.
Former cabinet minister Esther McVey tweeted: “Pleased the Cabinet and PM (recognising where majority opinion is in parliamentary party) are now listening to their backbench MPs and for once pushed back on the scaremongering by the lockdown fanatics.
“It seems the 100 strong backbench rebellion last week has made a difference.”
However Labour, which has said it would support the Government over any further public health measures that were needed, accused the Prime Minister of putting party interests before the country.
Shadow health secretary Wes Streeting said: “Boris Johnson is too weak to stand up to his own backbenchers, many of whom have no plan beyond ‘let the virus rip’.
“Today, while businesses across the country wonder if they can continue to trade, and families make frantic calls about whether they will see each other this Christmas, true to form the Prime Minister has put his party before the public.
“Rather than set out a clear plan for the country, he has chosen to protect himself from his own MPs by simply not saying anything. Boris Johnson is unfit to lead.”
Liberal Democrat leader Sir Ed Davey said the Mr Johnson had offered only “chaos” at a time when the country needed clarity.
“Families want to know urgently what Covid measures to expect, so they can plan for the days ahead. Ducking the difficult decisions is not a plan,” he said.
Meanwhile, it emerged that the Queen had decided to spend Christmas in Windsor rather than to travel to Sandringham in what one royal source said was a precautionary approach.
The latest Government figures showed there were a further 91,743 lab-confirmed Covid-19 cases in the UK as of 9am on Monday, with a further 44 deaths of people who had died within 28 days of testing positive for the disease.
The UK Health Security Agency (UKHSA) said there had been 8,044 additional confirmed cases of the Omicron variant reported across the UK, bringing the total number confirmed cases to 45,145.
Earlier, Downing Street did not deny reports that a number of ministers – including Chancellor Rishi Sunak – have pushed back against calls for action without more evidence of the impact the virus would have.
The Prime Minister’s official spokesman said it was part of the job of ministers to scrutinise any advice they were given.
“We need to strike the right balance between protecting lives and livelihoods. That is what we are focused on,” the spokesman said.
“It is one of the roles of ministers to scrutinise any advice and evidence provided, and consider it in the round. That is the function of Cabinet.”
The Government has said that it will if necessary recall Parliament to allow MPs to vote on any new regulations it proposes for England.
However, some MPs have questioned whether he would risk another backbench revolt at a time when he is politically weakened by ongoing row over Downing Street parties and the Tories’ crushing defeat in the North Shropshire by-election.