Britons are being urged to “keep their cool” during a nationwide test of the emergency alert system this Sunday.
At about 3pm, mobile phones connected to 4G and 5G networks will emit a distinctive siren sound and vibrate for up to 10 seconds.
Users will receive a message confirming the alert is a drill.
This is the second test of the system. The first was in 2023.
The government has previously deployed the system five times for genuine warnings, including during January’s Storm Eowyn, to alert residents in Scotland and Northern Ireland to severe weather conditions.
Approximately 3.5 million people across Wales and southwest England received an alert during Storm Darragh last December.
Meanwhile, a 500kg unexploded Second World War bomb found in a Plymouth back garden triggered a warning to some 50,000 phones in February 2024.
Messages can be targeted to relatively small areas to pinpoint those at risk.
About 15,000 phones were alerted during flooding in Cumbria in May 2024, and 10,000 received a warning during flooding in Leicestershire in January 2025.
The system is designed for use during the most likely emergencies to affect the UK. Warnings would also be transmitted on television, radio and locally by knocking on doors.
Government officials have also met with domestic violence charities and campaigners for discussions on helping those who may need to opt out of the test.
Pat McFadden, work and pensions secretary, said: “I know Brits will keep their cool when phones across the UK make a siren-like noise [on Sunday] at 3pm. It’s important to remember this is only a test, just like the fire drills we all do in our schools and workplaces.
“We’re carrying out the test to make sure the system works well when we need it most, and afterwards, we’ll work with mobile network operators to assess performance.
“The test takes just 10 seconds, but it helps us keep the country safe 24 hours a day, 365 days a year.”
