Millions of mobile phones across the UK will emit a loud alarm and vibrate in a nationwide test of a new public alert system later this month. On Sunday, 23 April, a siren will go off on nearly every smartphone in the UK at 15:00 BST to test a new emergency alerts system.
The test had originally been planned for the early evening but was moved to avoid clashing with an FA Cup semi-final, which kicks off at 16:30, and the London Marathon, which starts at 09:30 on that Sunday.
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.
The system is intended to be used in life-threatening situations, and it is modeled on similar schemes in the US, Canada, the Netherlands and Japan. Thirty years after Britain decommissioned its national air raid siren network, the government is setting up a new system to warn citizens in the event of an emergency — only this time over mobile phones.

The minister in charge of the system, Oliver Dowden, said it would be used only in situations where there was an immediate risk to life. The alert message and alarm is expected to hit 90% of mobile phones in the UK. In most cases it will be targeted at very specific areas, rather than the entire country and, according to officials, may not be used for months or years.
Phone users can swipe away the alert message or click “OK” on their home screen to continue using their phone as normal. People who have their phones switched off will not receive the message – but it will sound if your phone is switched to silent. People who do not wish to receive the alerts will be able to opt out in their device settings, but officials hope the life-saving potential of the messages means that users will keep them on.
What will the message say?
The Alert will contain the message in both Welsh and English. It will say: “Prawf ar Rybuddion Argyfwng yw hwn, sef gwasanaeth newydd gan lywodraeth y DU a fydd yn eich rhybuddio pan fydd argyfwng sy’n berygl i fywyd gerllaw.
“Mewn argyfwng go iawn, dilynwch y cyfarwyddiadau yn y rhybudd i’ch cadw chi ac eraill yn ddiogel. Ewch i gov.uk/alerts i gael rhagor o wybodaeth. Prawf yw hwn Does dim angen i chi wneud dim.
“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 an actual 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.”
The government said that a UK-wide test of the Emergency Alerts system would take place at 3 pm BST on April 23. The “broadcast” system could also be used to warn about security threats, including terrorist attacks, as it is in other countries, such as the Netherlands. National Fire Chiefs Council chairman Mark Hardingham said that for 10 seconds, the national test may be inconvenient for some, but he urged people to forgive the intrusion because, the next time they hear it, their life, and the life-saving actions of the emergency services, could depend on it.

The government has also tried to play down concerns that drivers will be distracted by the alerts, potentially leading to accidents, saying evidence from local trials of the alert shows people will wait until they are stationary to check their phones. The National Centre for Domestic Violence (NCDV) warned people with hidden second mobile phones to turn off the alerts to avoid revealing the location of their devices.
All 4G and 5G Android and Apple phones are already fitted with emergency alert capability, as similar systems are in use in the United States, Canada, Japan and other countries around the world. There is no need for users to register their number.