Millions of mobile phone users across the UK will receive a loud emergency alert as the government prepares to test its national warning system later this year.

Phones will emit a siren-like sound lasting up to 10 seconds, even if set to silent. A message will appear on screen and must be acknowledged before the phone can be used again. It may include official advice, phone numbers or links to government websites.

The Emergency Alerts system, launched in 2023, is designed to warn the public about life-threatening events such as severe flooding, fires, or extreme weather. Only emergency services, government departments and public agencies can issue these alerts.

The upcoming alert will be the final national test of 2025, with future tests to take place every two years. The exact date has not yet been announced, but the government has confirmed that more details will follow in due course.

The system was introduced following recommendations from the Covid-19 Inquiry, which highlighted the lack of direct public warning tools during national emergencies.

Alerts are sent via 4G and 5G networks and are expected to reach about 90% of mobile phones in a given area. Devices that are switched off, in flight mode, or using only WiFi or 2G/3G networks will not receive the alert. Users can also choose to opt out via their device settings.

A similar alert was previously used during Storm Darragh in December 2024, when around three million people in Wales and South-West England were warned of life-threatening weather.

Emergency alert systems are already widely used in countries including the US, Canada, Japan, and the Netherlands, especially during natural disasters.