A “national warning message” will be sent directly to every mobile phone across Britain later this month or in early March.
Before the alert is sent a publicity campaign will warn people to expect it and not to panic when they get it.
Officials say the test may be the only time a nationwide message is sent but it will demonstrate that the new system is capable of reaching every part of the country.
Alerts can also be limited to specific areas by controlling which mobile phone masts they broadcast from. In practice the system is expected to be used for local “extreme weather warnings” such as flood alerts.
The Government previously sent out a national text message at the start of the Covid crisis in March 2020, ordering people to stay at home, but this was only possible with assistance from mobile phone operators. Rather than sending a text, the new emergency alert uses an app included in Android and Apple iPhone operating systems.
This can be turned off in phone settings, although it is thought many phone owners won’t even know they have it installed.