The UK's coronavirus alert level has been increased from level three to four - meaning transmission of cases is high or rising exponentially.
The last time the UK was at this level was between late February and May this year.
Why has the alert level gone up?
A joint statement from the UK's four chief medical officers, and NHS England director Professor Stephen Powis, said the change was "in light of the rapid increase in Omicron cases".
The statement says: "Transmission of Covid-19 is already high in the community, mainly still driven by Delta, but the emergence of Omicron adds additional and rapidly increasing risk to the public and healthcare services."
It added that early evidence shows Omicron was "spreading much faster than Delta and that vaccine protection against symptomatic disease from Omicron is reduced".
"Hospitalisations from Omicron are already occurring and these are likely to increase rapidly."
The Covid alert level system is separate and independent from any government decisions on easing or tightening restrictions.
How are the levels set?
Risk levels are measured by a five-level, colour-coded alert system.
The government unveiled the system in May 2020.
There are five levels:
- Level five (red) - a "material risk of healthcare services being overwhelmed"
- Level four - a high or rising level of transmission
- Level three - the virus is in general circulation
- Level two - the number of cases and transmission are low
- Level one (green) - Covid-19 is no longer present in the UK
What determines the level?
- Covid-19's reproduction (R) number, a scientific measure of how fast the virus is spreading
- The number of confirmed coronavirus cases at any one time
Who sets the level?
The Joint Biosecurity Centre (JBC) - set up by the government in the spring of 2020 - has the task of recommending what the alert level should be.
JBC scientists identify changes in infection rates using testing, environmental and workplace data.
The JBC also has an "insight team" which monitors local spikes of Covid-19 and advises health officials and local authorities.
Their recommendations are then reviewed and agreed by the chief medical officers of the four UK nations.
Does a change of level mean that restrictions are eased or tightened?
Not automatically. The Covid-19 alert level system is separate and independent from any government decisions on easing or tightening restrictions.