The UK's coronavirus alert level is being upgraded from three to four.
It means transmission of the virus is now high, or rising at an increasing rate.
How is the alert level set?
Risk levels are measured by a five-level, colour-coded alert system.
The government unveiled the system in May. The prime minister said it would help decide how tough social-distancing measures should be.
There are five levels:
- Level five (red) - a "material risk of healthcare services being overwhelmed" - extremely strict social distancing
- Level four - a high or rising level of transmission - enforced social distancing
- Level three - the virus is in general circulation - social distancing relaxed
- Level two - the number of cases and transmission are low - minimal social distancing
- Level one (green) - Covid-19 is no longer present in the UK - no social distancing
What is the current level?
The alert level was reduced from level four to three on 19 June, but is now being put back up to level four.
That means the epidemic is in general circulation and transmission is "high or rising exponentially".
The change was announced on 21 September by the chief medical officers of England, Scotland, Wales and Northern Ireland.
In a statement they said cases were "rising rapidly and probably exponentially in significant parts of all four nations".
They said that to avoid significant excess deaths and exceptional pressure on the NHS "everyone has to follow the social distancing guidance, wear face coverings correctly and wash their hands regularly".
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 May - 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. In England, those decisions are determined by the government's five tests.
However, the alert level system is used by the UK governments to help their decisions on lockdown.