Millions of phones across the country will emit a loud noise and vibrate for about 10 seconds at 3pm to trial the life-saving system.
The alert, revealed by this newspaper and modelled on ones in the US and Canada, will be used in life-threatening situations such as flooding and wildfires.
As well as an alarm sounding, a text message 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.”
Users will be prompted to acknowledge the alert by clicking the message.
Mr Dowden, who replaced Dominic Raab as Deputy PM on Friday, said: “This is no different to testing a fire alarm.
“It’s a bit of a nuisance – but you wouldn’t want to leave it until a real-life emergency to test it for the first time.”
Only 32 percent planned to opt-out or switch their phones off. With several major sporting events taking place on Sunday, spectators have been warned the test could briefly disrupt their viewing.
Organisers of the World Snooker Championship at the Crucible in Sheffield will pause play just before 3pm to allow for the alert.
The test also coincides with the later stages of the London Marathon and the end of half-time in the 2pm kick-off Premier League ties at Bournemouth and Newcastle United. Theatre and cinema-goers will be told to switch off their phones before shows start.
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.