The Covid alert level has been raised from Level 3 to Level 4.
The move comes following a rapid increase in Omicron cases being recorded throughout the UK.
The four chief medical officers and NHS England’s national medical director have recommended the change to ministers.
The decision to increase the alert level follows advice from the UK Health Security Agency (UKHSA) after a further 1239 confirmed cases of the Omicron mutation were recorded in the UK as of Sunday.
It brings the total number of UK cases of Omicron to 3137, a 65% increase from Saturday’s total of 1898 UK cases.
Scotland also saw its biggest jump in Omicron cases since the new variant was identified.
On Sunday, the Holyrood government confirmed there were 159 cases of Omicron across the country, more than triple last week’s number.
Boris Johnson is preparing to address the nation at 8pm about the booster vaccine programme.
Nicola Sturgeon revealed on Sunday that the booster will be available to 30-39-year-olds in Scotland from Monday.