Britain is planning to test its public emergency alert system across all mobile phone networks later this month.
Millions of people across the UK will receive a "siren-like" warning on their phones which would provide information about how to stay safe in the event of a potential emergency.
Here's what we know about the plans so far:
What is it?
On Sunday, April 23, mobile phones and some tablets across all four UK nations will emit a ten-second blast to test the new public alert system.
The alerts will appear on the home screens of people’s devices, accompanied by a loud warning sound and vibration.
Up to 90 per cent of phones are expected to receive the alarm which will sound even if their device is on silent. Devices that are turned off, in airplane mode or connected to 2G or 3G networks will likely not receive the message.
Phone users will be unable to use other features on their devices unless they acknowledge the alert.
Why is it being introduced?
The system – modelled after similar schemes in the US, Canada, the Netherlands and Japan – is intended to be used in life-threatening situations including flooding and wildfires.
It has previously been used by the UK's Environment Agency to warn about the threat of flooding, but will now be rolled out across Britain.
The UAE uses a similar system for alerts about extreme weather events, while some American states use the alert system to send notifications about natural disasters and missing children.
Lord Harris, who is chairman of the National Preparedness Commission, claims that "many lives" could have been saved during the Grenfell Tower disaster had a similar system been in place.
"The fire commander simply had no means of communicating with the residents of the block living 20 floors above the fire. That system could have done that for him", he told the BBC's Today programme.
Why are people worried?
Charities and other public interest groups have raised numerous concerns about the forthcoming tests.
The RAC has warned that the alarm will be a major distraction for unsuspecting motorists and could put road users in danger.
The UK government advice states drivers should not attempt to respond to the noise until they have stopped their vehicle in a safe location.
Women's charities also say they are concerned that the alarm could reveal the whereabouts of second phones belonging to vulnerable domestic violence victims.
Can it be switched off?
Users can opt out of the alerts in their phone's system settings, although some say this varies by device and is not entirely straightforward.
Government advice states that people should keep the alerts switched on for their own safety and claims that no data will be collected or shared.
