A siren-like alert will be sent to mobile phone users across the UK next month to test a new government public warning system.
It allows the government and emergency services to send urgent messages warning the public of life-threatening situations like flooding or wildfires.
A message will appear on the home screens of people's devices during the test, with vibration and a loud warning sound that will ring for about 10 seconds - even if the phone is set to silent.
Messages would only ever come from the government or emergency services and will initially focus on the most serious weather-related events, with the ability to get a message to 90% of mobile users within the relevant area.
They will only be sent where there is an immediate risk to life and many people may not receive an alert for months or years.
People can opt out by searching their device settings for emergency alerts and then turning off severe and extreme ones. Officials say the alerts could be life-saving, though, advising against switching them off.