Monday Dec 13, 2021
This was at the recommendation of the country's chief medical officers.
"Early evidence shows that Omicron is spreading much faster than Delta and that vaccine protection against symptomatic disease from Omicron is reduced," the government said in a statement.
The upgrade to level four means the health authorities think that the rates of transmission are high and putting pressure on healthcare services.
Last week Johnson introduced "Plan B" COVID-19 measures for England, ordering people to work from home, wear masks in public places and use vaccine passes to slow the spread of Omicron.
The seven-day average of COVID cases by date reported has risen in recent days above 50,000 - the highest since the January peak during the last wave of cases.
With 146,439 fatalities recorded within 28 days of a positive COVID-19 test as of Sunday, Britain has Europe's highest death toll from the virus.
Britain faces a "tidal wave" of the Omicron variant of coronavirus and two vaccine doses will not be enough to contain it, Prime Minister Boris Johnson warned on Sunday, as he accelerated the booster rollout programme.
Speaking hours after government scientists lifted the COVID alert level to 4 on a 5-point scale, Johnson said the booster programme must go faster because scientists did not yet know if Omicron was less severe than other variants.
"A tidal wave of Omicron is coming," Johnson said in televised statement on Sunday evening. "And I'm afraid it is now clear that two doses of vaccine are simply not enough to give the level of protection we all need."
He added that scientists knew Omicron was much more transmissible and that the National Health Service would struggle to cope with hospitalisations if the variant were to tear through an un-boosted population.
"Everyone eligible aged 18 and over in England will have the chance to get their booster before the New Year," he said.