Loud siren-like sounds are to be triggered on mobile phones in April when the government runs a test of its new national emergency alerts system.

While fully operational the service - nicknamed the Armageddon alarm - will broadcast alerts and send emergency advice to people's devices to warn them of nearby danger that could threaten their life such as a large fire, flooding or a terrorist attack.

But as part of the service's roll-out, the first nationwide trial of the alarm system is to take place in April.

Here's everything you need to understand about the test.

When is the alarm test happening?

After much speculation that officials had been gearing up to run nationwide tests of the emergency alarm system, ministers have now confirmed the first of those will take place on Sunday, April 23 in the 'early evening'.

The government does not need mobile phone numbers in order to trigger alerts because the warnings are broadcast from phone masts and are automatically picked up by devices within range.

The alert, which it is triggered, will set off a "loud siren-like sound", vibrations and convey a message on the screen in a routine lasting around 10 seconds at a time.

Even if phones are set to silent the message is expected to still sound but you will not receive the messages and sounds if your phone is switched off or in aeroplane mode.

What will the alert say?

If the alarm system ever needs to be triggered in the event of a real emergency, information displayed on the screens of devices within range will outline the information and instructions people need to take on board and follow.

When it is eventually rolled-out it is understood emergency planners will first use the system to alert to flooding - but ministers are quick to point out it could be many months or even years before people actually pick up a message for a real issue.

However April's test, says the government, will make it clear to mobile phone users who receive the alert that the event is indeed just a test run and nothing to panic about. It will last for up to 10 seconds.

Previous tests held in Reading in 2021 sent people a short message which read: "This is a mobile network operator test of the Emergency Alerts service. You do not need to take any action. To find out more, search for gov.uk/alerts."

You will however, need to acknowledge the message sounding before you'll be able to use any further features on your phone just as you would a 'low battery' message.

Do not disturb

Setting your phone to silent when the alarm is triggered could see it sound anyway - you'll need to turn your device off or switch to aeroplane mode to guarantee not receiving the test alert.

While the government is trying to disuade phone users from opting-out people who don't wish to be disturbed, or who would only wish their phone to be triggered in the case of a real and severe emergency, may be able to turn off some notifications as the system is steadily rolled out to devices nationwide.

You will however, not be able to shut down the most important warnings which come with a risk to life, when the system becomes fully operational, suggests the government website.

The warning says: "You can opt out of some emergency alerts, but not the most important ones. You cannot opt out by subject, only by how serious the emergency is. If you opt out because you do not want flood warnings, for example, you might miss alerts for fires and terrorism.

"Because of this, you should keep emergency alerts switched on for your own safety."

Opting out

Smart phone owners can see if their device is already programmed with the new system by searching their phone's settings for 'emergency alerts'.

If your phone has the available settings you'll be able to select and deselect on which levels you'd wish your phone to sound an alarm if it is ever triggered locally.

Options currently displayed within the settings on some Android devices include Amber, Severe and Extreme - albeit the government insists switching off the most severe level of alert actually won't necessarily always be possible.

Some Android users who were part of the initial test in Reading two years ago also reported being able to switch-off the trial with an option on their device to 'turn off test alerts'.

However, these alerts are potentially life-saving, says the government, so it recommends leaving all options switched on at all times.