Cases of the Omicron variant in people without any travel history have been confirmed in the UK meaning it is now being transmitted within the community, Health Secretary Sajid Javid said.
Mr Javid said there are now 261 Omicron variant cases in England, 71 in Scotland and four in Wales – a total of 336.
“This includes cases with no links to international travel so we can conclude there is now community transmission across multiple regions of England,” he told the Commons.
The health secretary said he could not guarantee the variant would not “knock us off our road to recovery”.
He added the UK Health Security Agency (UKHSA) now has data that shows the Omicron variant may have a shorter window than other variants between infection and showing symptoms.
He said from 4am tomorrow, anyone travelling to the UK from a non-red list country, whether vaccinated or not, will have to show proof of a negative PCR or lateral flow test taken no earlier than 48 hours before departing for the UK.
Mr Javid added that the new measures, which also include every traveller having a day 2 PCR test and isolating until getting a negative result, are temporary and will be reviewed next week.
“We must make sure measures are absolutely necessary, we won’t keep measures in place for a day longer than we have to,” he said.
He added that due to concerns over increased PCR testing costs, more than 100 providers have been removed from the government’s website in recent weeks for “showing misleading prices and we will continue to take a tough and hard line”.
The health secretary also said he expects the number of hotel rooms available for international quarantine to double this week as he acknowledged UK and Irish residents been unable to return from countries placed on the red list last week due to a lack of hotel quarantine rooms.
“We are ramping up this capacity as quickly as possible. We’ve already brought several new hotels on board in the past few days and we expect to double the number of rooms that are available this week,” he said.
People who are not UK or Irish residents are not allowed to travel to the UK from red list countries, which now include Nigeria as well as several southern African nations including South Africa where the variant was first identified.
Mr Javid addressed Nigeria being added to the red list from 4am on Monday, saying UKHSA analysis shows “at least 21 Omicron cases in England alone are linked to travel from Nigeria”.
He added: “And there’s a strong indication that Omicron is present there. Nigeria also has very strong travel links with South Africa.”
Moments before Mr Javid spoke, the UKHSA reported there had been a further 90 cases of the Omicron variant over the past day in the UK, bringing the total to 336.
On Sunday, there were 86 new cases of the variant reported.
The UKHSA said 64 of the latest cases were in England, 23 in Scotland and three were identified in Wales. There have been no Omicron cases reported in Northern Ireland so far.
Experts have said there is increasing concern Omicron is spreading more quickly than the Delta variant, and there are probably more than 1,000 cases in the UK already.
Professor Paul Hunter, from the school of medicine at the University of East Anglia, said the early signs show “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,” he told BBC Breakfast.
England’s deputy chief medical officer Professor Jonathan Van-Tam said the “number of mutations present, already on first principle, makes us worry about a possible effect on vaccine effectiveness”.
Nigeria is the latest country to be placed on the red travel list after several southern African nations were put on it last week, meaning arrivals must now isolate in a quarantine hotel.
Any travellers entering the UK must now take a pre-departure test and get a private PCR test on day two after arriving, and isolate while waiting for the result.