The UK’s chief medical officers have agreed to downgrade the coronavirus alert level from four to three following a 'steady and continuing' decrease in cases in all four nations.

Localised outbreaks of Covid-19 are still 'likely' to occur, the advisers warned - and the virus remains in general circulation.

But the downgrading – recommended by the Joint Biosecurity Centre (JBC) – means transmission of coronavirus is no longer considered to be “high or rising exponentially”.

England’s Professor Chris Whitty, Northern Ireland’s Dr Michael McBride, Scotland’s Dr Gregor Smith and Wales’ Dr Chris Jones agreed with the JBC’s recommendation.

But what is the alert system and how does it work?

Here's what you need to know.

What is the alert system?

The Covid Alert Levels system was announced by Prime Minister Boris Johnson in his televised address to the nation on May 10.

The system is similar to that used to establish the terrorist threat and will be run by a new Joint Biosecurity Centre.

It has five tiers from level one to five based on the spread of Covid-19 through the country.

Bromsgrove Advertiser: The Covid-19 alert level system. Picture credit: PA GraphicsThe Covid-19 alert level system. Picture credit: PA Graphics

How does it work?

Mr Johnson said that the country’s current alert level will be determined primarily by the virus’ reproduction rate – the average number of people that will contract coronavirus from an infected person – and the number of coronavirus cases.

The level will then inform the Government what measures are needed to combat the spread of the virus, with higher levels requiring stricter controls.

What are the different levels?

When the scheme was launched the country was at level four which means a “Covid-19 epidemic is in general circulation, transmission is high or rising exponentially”.

The system says that current social distancing measures and restrictions should remain in place.

Level five is when transmission is high or rising but also when there was a risk of healthcare services being overwhelmed and will require an increase in social distancing measures.

Level one means there Covid-19 is no longer known to be in the UK and the only action should be “routine international monitoring”.

Level three is when the epidemic is in general circulation and gradual easing of restrictions can take place, while level two is when the number of cases and transmission is low and “no or minimal” restrictions are required.

But under level two there will be enhanced testing, tracing, monitoring and screening.

Who monitors and sets the level?

Mr Johnson said he was establishing a new Joint Biosecurity Centre to run the alert system.

Cabinet Office documents said that the centre will provide real time analysis and assessment of Covidd-19 outbreaks at a community level to enable rapid action to stop spikes in infection.

It will also advise on specific actions which can be taken to manage rising numbers of infections, such as closing schools or businesses.

The centre will also inform the chief medical officers of a change in the Covid-19 Alert level who will then advise ministers.