A huge-scale warning system has been tested by the UK government, sounding an alarm on nearly all mobile devices across the country.

‘Emergency Alerts’ is a new government service designed to issue warnings quickly to large numbers of people. It was tested at 3pm on Sunday 23 April, with a warning message displaying on all 4G- and 5G-enabled devices that 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. This is a test. You do not need to take any action.”

As well as the message a loud sired played – even on devices set to silent. However, the warning didn’t reach devices that were switched off or on aeroplane mode, or those running older Operating Systems than iOS 14.5 or Android 11.

It was forecast that 90% of mobile devices would receive the notification, although large volumes took to social media after the event to claim they had either not received a notification, or that it came a minute or two earlier than anticipated.

The alerts were broadcast from mobile masts to all devices within their range. No personal data was taken and neither the government nor emergency services needed to know the mobile numbers of devices that received a warning.

That said, anyone concerned about the warning is able to opt out of receiving future notifications, though the government doesn’t recommend this. To do this, simply switch off notifications for severe and extreme alerts in the ‘Emergency Alert’ settings.

Alerts are expected to be used most often for extreme weather-related events, such as flooding or fire.

Warning systems such as this are already in wide use across the world, with the USA, Netherlands and Japan among those with schemes already set up. Japan is one of the biggest advocates of the system, saying it has been responsible for saving “countless” lives during natural disasters.