At 15:00 BST, millions of phones across the UK will sound a loud siren and vibrate for about 10 seconds as part of a national emergency alert test.
The alerts are designed to warn people of life-threatening events like severe weather or terror attacks.
Messages will appear on compatible 4G and 5G phones, but not if devices are off, on airplane mode, or only connected to Wi-Fi.
Work and Pensions Secretary Pat McFadden urged people not to panic, calling it just like a fire drill to check the system works.
The service is accessible for deaf and blind users and has already been used during major storms and emergencies, including the Plymouth bomb evacuation in 2024.
The test message will read:
“This is a test of Emergency Alerts… You do not need to take any action.”
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