At 15:00 BST today - compatible phones will vibrate and make a siren sound for 10 seconds, even if set to silent.
They will also display a message explaining that a test is taking place.
The alerts are intended for situations in which there is an imminent danger to life, such as extreme weather events or during a terror attack.
Pat McFadden, the new work and pensions secretary, said the test is "to make sure the system works well when we need it most".