The Omicron variant of Covid-19 is now spreading in “multiple” parts of England, health secretary Sajid Javid told MPs on Monday.
In a statement, the Mr Javid confirmed that community transmission of the variant was happening across the country.
He said the 336 Omicron cases detected so far includes some with no links to international travel.
“So we can conclude that there is now community transmission across multiple regions of England,” Mr Javid told MPs.
Some 52 countries have now reported the presence of the new variant, with 11 - including Romania, Mexico and Chile - declaring their first cases this weekend, the health secretary said.
His update follows a decision over the weekend to require pre-departure tests for people travelling to the UK as part of the effort to contain the Omicron variant.
Mr Javid said he could not guarantee the variant would not “knock us off our road to recovery”, as he said the “the window between infection and infectiousness may be shorter for the Omicron”.
He told BBC Breakfast: “How it’s likely to spread in the UK still uncertain, but I think the early signs are that it will probably spread quite quickly and probably start outcompeting Delta and become the dominant variant probably within the next weeks or a month or so at least.
“The big remaining question is actually how harmful it is if you do get Covid with this Omicron variant, and that’s the question that we’re struggling to answer at the moment.”
But he said that as far as he was aware, none of the 336 people with a confirmed case of Omicron in the UK had been admitted to hospital.
He said: “We are learning more about this new variant all the time.
“Recent analysis from the UK Health Security Agency suggests that the window between infection and infectiousness may be shorter for the Omicron variant than for the Delta variant, but we don’t yet have a complete picture of whether Omicron causes more severe disease or indeed how it interacts with the vaccines.
“We can’t say for certain at this point whether Omicron has the potential to knock us off our road to recovery.”
He added: “We are leaving nothing to chance. Our strategy is to buy ourselves times and to strengthen our defences while our world-leading scientists assess this new variant and what it means for our fight against Covid-19.”
It also said that a further 41 people had died within 28 days of testing positive for Covid-19.
Separate figures published by the Office for National Statistics show there have now been 170,000 deaths registered in the UK where Covid-19 was mentioned on the death certificate.
Mr Javid told MPs the Government was taking “early action now so we don’t have to take tougher action later on”.
He said: “When this new variant is appearing in more and more countries every day we also need to look beyond the red list and strengthen our measures for a wider range of travellers to make sure they give us the protection we need against this potential threat.”
However, former Conservative prime minister Theresa May warned about the impact of responding to new variants by “stopping and starting sectors of our economy”.
She said: “The early indications of Omicron are that it is more transmissible but potentially leads to less serious illness than other variants.
“I understand that would be the normal progress of a virus. Variants will continue to appear year after year.
“When is the Government going to accept that learning to live with Covid, which we will all have to do, means we will almost certainly have an annual vaccine and that we cannot respond to new variants by stopping and starting sectors of our economy which leads to businesses going under and jobs being lost?”
Mr Javid replied: “In terms of the severity of this, I think we shouldn’t jump to any conclusion, we just don’t have enough data.
“It is not going away ... for many, many years and perhaps it will lead to annual vaccinations, but we have to find ways to continue with life as normal.”