Millions of mobile phones across the UK will ring out a loud alarm and vibrate in a nationwide test of a new public alert system later this month.
The system is intended for use in life-threatening situations such as floods and forest fires.
But domestic violence activists have warned the test could put people at risk by revealing the location of secret phones hidden 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 are prompted to acknowledge the alert by swiping or clicking the message before continuing to use their device.
The government has now set the exact time for the test – on Sunday 23 April at 3pm.
The St George’s Day Test coincides with big events including the London Marathon and the 2pm Premier League games between Bournemouth and West Ham and Newcastle and Tottenham Hotspur.
Officials said they have been working with the Football Association and organizers of the marathon to ensure the impact of the test remains limited.
National Fire Chiefs Council leader Mark Hardingham said: “For 10 seconds the national test may make some uncomfortable but please forgive us for intruding because next time you listen it is about your life and them life-saving measures of our emergency services could depend on it.”
The system is modeled after similar systems in the US, Canada, the Netherlands and Japan.
The Chancellor of the Duchy of Lancaster, Oliver Dowden, said: “Commissioning this system with the national test means we have another tool in our toolkit to help protect the public in life-threatening emergencies.
“It could be the sound that saves your life.”
People who don’t want to receive the notifications can opt out in their device settings, but officials are hoping the messages’ life-saving potential means users keep them on.
The National Center on Domestic Violence (NCDV) warned people with hidden second cell phones to turn off the alerts to avoid revealing the location of their devices.
NCDV’s Sharon Bryan said: “Hidden second phones are a lifeline for victims and survivors who are constantly at risk of abuse or worse.
“This siren test can unexpectedly reveal your presence to perpetrators – with catastrophic consequences.”
The government said it has been actively working with organizations working with vulnerable women and girls to ensure they are not affected by the rollout of emergency alerts.
Officials stressed that if people need to keep their phones hidden, it’s easy to opt out of the system by either turning off the alerts or simply turning off the phone during the test.
Deputy Chief of Police Owen Weatherill of the National Police Chiefs’ Council said: “Together with our partners, we will continue to listen carefully to public feedback and ensure that the use of emergency alerts is having a positive impact.”