The test alert, which will take place at 3pm on Sunday 23 April, will see people receive a message on the home screen of their mobile phone, along with a sound and vibration for up to ten seconds. The 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.

https://www.gov.uk/alerts for more information