A message with a siren at 15:00 BST on 23 April will say "in a real emergency, follow the instructions in the alert to keep yourself and others safe".
It will give a sound and vibration for 10 seconds even for phones on silent.
Phone users will have to acknowledge the alert before they can continue using their devices.
The alert system will be used to warn of extreme weather events, such as flash floods or wildfires. It could also be used during terror incidents or civil defence emergencies if the UK was under attack.
It is being sent to 4G and 5G mobile phones.
The full message will read: "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."
Drivers are being advised not to look at or touch their phone until it is safe to do so, just as when receiving calls or messages.
"Getting this system operational means we have a vital tool to keep the public safe in life-threatening emergencies," said Oliver Dowden, the minister in charge of the system.
"It could be the sound that saves your life," he said.
Alex Woodman, from the National Fire Chiefs' Council, called on the general public to "play their part" in helping to keep people safe.
"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," he said.
According to an instructional video issued by the government, the alerts will only ever be sent by the emergency services or the government, and the same message will go to all compatible phones within an area of risk. They do not use phone numbers, collect data or track movements, the video says.
There is no need to register or download an application.
All 4G and 5G Android and Apple phones are already fitted with emergency alert capability, as similar systems are in use in countries including the US, Canada and Japan.
However domestic abuse campaigners, including the charity Refuge, have warned the test could put some vulnerable people in danger by alerting an abuser to the location of a secret phone.
The government said it had been engaging with such organisations to ensure those at risk were not adversely affected by the introduction of emergency alerts.
Officials also said it was possible to opt out of the system if people needed 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 had originally been planned for the early evening but was moved to avoid clashing with the FA Cup semi-final between Manchester United and Brighton.