Millions of mobile phones across the UK will emit a loud alarm and vibrate 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.
But 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 message will be received on 4G and 5G mobile phones, along with sound and vibration for up to 10 seconds.
Phone users will be prompted to acknowledge the alert by swiping or clicking the message before being able to continue using their device.
Chancellor of the Duchy of Lancaster Oliver Dowden said: “Getting this system operational with the national test means we have another tool in our toolkit to keep the public safe in life-threatening emergencies.
“It could be the sound that saves your life.”
The system is modelled on similar schemes in the US, Canada, the Netherlands and Japan.
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.
People who do not wish to receive the alerts will be able to opt out in their device settings, but officials hope the life-saving potential of the messages means that users will keep them on.
The National Centre for Domestic Violence (NCDV) warned people with hidden second mobile phones to turn off the alerts to avoid revealing the location of their devices.
NCDV’s Sharon Bryan said: “Hidden second mobiles are an emergency lifeline for victims and survivors living under the constant threat of abuse, or worse.
“This siren test may unexpectedly reveal their presence to abusers – with disastrous consequences.”
Officials stressed that it is easy to opt out of the system if people need their phone to stay concealed, either by turning off the alerts or simply having the phone switched off during the test.
Assistant Chief Constable Owen Weatherill, from the National Police Chiefs’ Council, said: “Alongside partners, we will continue to listen carefully to public feedback and ensure the use of emergency alerts has a positive impact.”