This weekend the Government will be testing a new emergency alert system.
The test will happen on Sunday afternoon (23 April) at 3pm.
Something like this might appear on your mobile phone or tablet:
The test will be sent to every mobile phone and some tablets that are switched on at 3pm on Sunday.
The sound and vibration will last for 10 seconds.
If you want to stop the alert sound and vibration you can press OK or swipe across the screen.
If your phone or tablet is turned off the test will not sound or vibrate.
You can read more about the emergency alert system on Gov.uk. Click here to read about it.