Millions of phones will receive emergency alerts on 7 September in the second national test of the warning system.
Smartphones will vibrate and sound a 10-second siren at 3.00pm on the Sunday in a test of the alert, which is designed to warn of nearby threats such as extreme weather or terror activity.
Even phones on silent mode will light up during the alert, while a message will be displayed on the screen showing it is a test.
There are approximately 87 million mobile phones in the UK, according to officials.
The test will be the first since April 2023 and the government said it is part of plans to strengthen the country’s emergency preparedness. The Covid inquiry recommended that ministers test the Emergency Alert System in the wake of the pandemic.
Concerns have been raised in the past that emergency alerts can leave victims of domestic abuse at risk, by exposing those who have secret phones. But the government said it will run a public information campaign targeted at vulnerable groups to ensure they can turn the alert off on second phones.
Chancellor of the Duchy of Lancaster Pat McFadden said: “Emergency alerts have the potential to save lives, allowing us to share essential information rapidly in emergency situations including extreme storms.
“Just like the fire alarm in your house, it’s important we test the system so that we know it will work if we need it.”
Since April 2023, the system has been used five times, including during major storms when lives were at risk.
Alerts flashed up on 4.5 million phones in Scotland and Northern Ireland during Storm Éowyn in January, and was seen by 3.5 million people in Wales and the South West of England received an alert during Storm Darragh last December.
It was also used when an unexploded bomb from the Second World War was discovered in Plymouth.
During the last test, a slew of mobile phone users, including those on the Three network, complained that they had not received the alert.
How to turn the alert off
It is easy to turn the feature off, but the system to do so depends on the device that is being used.
On an iPhone, open the Settings app and type “emergency alerts” in the search bar. This should bring up the option to turn off “severe alerts” and “emergency alerts”.
“In a real emergency, follow the instructions in the alert to keep yourself and others safe.
“Visit gov.uk/alerts for more information.”
On an Android device, go to the settings app and search for emergency alerts. The wording might depend a little on what version you are using, but it will usually be something like “emergency alerts”, and have the option to turn it off.
The British Red Cross welcomed the test but called for “tailored and targeted advice for people, especially those who may be more at-risk” of how to respond in the face of an emergency.
Chief executive Béatrice Butsana-Sita said: “The real test will be how this translates into tangible action, down to community and household level, so that we aren’t just warned but prepared and everyone is ready to respond in an emergency.”
And the Centre for Long-term Resilience said the UK’s national preparedness is “being tested to an unprecedented extent”, and welcomed the government’s recognition of that.
Users will have to acknowledge the alert, which will include telephone numbers or links to government websites with further information, before they can access any other features on their phones.
The alerts will not work if phones are switched off or in flight mode, or if people opt out of the scheme, and it is estimated that they will reach about 90 per cent of mobile phones in a given area, via 4G and 5G phone networks.
Previous alerts have 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.
Alert systems have been used successfully in a number of other countries, including the US, Canada, the Netherlands and Japan, during incidents such as severe weather events.
Separately, the government confirmed it is conducting a pandemic preparedness exercise dubbed Exercise Pegasus.