At least one person in the UK has died with the Omicron coronavirus variant......
Boris Johnson said the new variant was also resulting in hospital admissions, and the "best thing" people could do was get their booster jab.
Health Secretary Sajid Javid told MPs Omicron now represented 20% of cases in England.
Mr Johnson said people needed to recognise "the sheer pace at which [Omicron] accelerates through the population" and that they should set aside the idea that Omicron was a milder variant.
The prime minister tweeted that more than half a million people had booked their booster jab on Monday, which he described as an "incredible feat".The UK recorded 54,661 new coronavirus cases on Monday, as well as 38 deaths within 28 days of a positive test.
There are 4,713 confirmed cases of the Omicron variant but Mr Javid said the UK Health Security Agency estimated the current number of daily infections was around 200,000.Omicron has risen to more than 44% of cases in London and is expected to become the dominant variant in the city in the next 48 hours, he said.