Update:

A message with a siren at 15:00 BST on 23 April will say "in a real emergency, follow the instructions in the alert to keep yourself and others safe".

It will give a sound and vibration for 10 seconds even for phones on silent.

Phone users will have to acknowledge the alert before they can continue using their devices.

The alert system will be used to warn of extreme weather events, such as flash floods or wildfires. It could also be used during terror incidents or civil defence emergencies if the UK was under attack.

It is being sent to 4G and 5G mobile phones.

The full message will read: "This is a test of Emergency Alerts, a new UK government service that will warn you if there's a life-threatening emergency nearby.

"In a real emergency, follow the instructions in the alert to keep yourself and others safe."

"Visit gov.uk/alerts for more information.

The government will trial an emergency alert at 3pm and online shoppers are being asked not to shop or use their banking apps.

The Government will be testing its new smartphone Emergency Alert, popularly known as the ‘Armageddon Alert’ on April 23. Millions of mobile phone users across the UK will see loud alarms go off on devices, along with an alert message.

So best not to log on to anything important coming up to 3pm.