The system is intended to be used in life-threatening situations including flooding and wildfires.
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.
However, the test on St George’s Day coincides with major events including the Marathon and the 2pm kick-off Premier League ties between Bournemouth and West Ham and Newcastle and Tottenham Hotspur.
And domestic violence campaigners have warned the test could put people in danger by revealing the location of secret phones that have been hidden away by those at risk.