A message will appear alongside a loud alarm on millions of mobile phones across the UK at 3pm on April 23 in a nationwide test of a new public alert system.
The system is intended to be used in life-threatening situations including flooding and wildfires.
The Government has released the text that will pop up on people’s home screens next Sunday.
The message will be received on 4G and 5G mobile phones, along with sound and vibration for up to 10 seconds even if devices are on silent.
It will say: “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.”
Phone users will be prompted to swipe away the message or clicking “OK” on their home screen before being able to continue using their device.
Drivers are advised not to look at or touch their phone until it is safe, just as when receiving any call or message.
Chancellor of the Duchy of Lancaster Oliver Dowden said: “At 3pm next Sunday we’ll be doing a nationwide test of our new Emergency Alerts system.
“Getting this system operational means we have a vital tool to keep the public safe in life-threatening emergencies.
“It could be the sound that saves your life.”
Domestic violence campaigners have warned the test could put people in danger by revealing the location of secret phones hidden away by those at risk.
The Government said it has been actively engaging with organisations working with vulnerable women and girls to ensure they are not adversely affected by the introduction of emergency alerts.
Officials stressed that it is easy to opt out of the system if people need their phone to stay concealed, either by turning off emergency alerts in their settings or simply having the phone switched off during the test.
The test on St George’s Day coincides with major events including the London Marathon and the 2pm kick-off Premier League ties between Bournemouth and West Ham and Newcastle and Tottenham Hotspur.
Officials said they have worked with the Football Association and the Marathon’s organisers to make sure the impact of the test will be limited.
Chief fire officer Alex Woodman, from the National Fire Chiefs’ Council, said: “We must use every tool at our disposal to keep people safe, and we need everyone to play their part, and the new Emergency Alerts system is one way we can do this.
“For 10 seconds, the national test may be inconvenient for some, but it’s important, because the next time you hear it, your life, and the life-saving actions of our emergency services, could depend on it.”