The Trump administration will send a message to all US mobile phones on Thursday, as it tests an unused alert system that warns the public about national emergencies.
Phones will make a loud tone and have a special vibration according to the Federal Emergency Management Agency (FEMA), which will send the alert.
The test message will be headlined “Presidential Alert” and will go on to read “THIS IS A TEST of the National Wireless Emergency Alert System. No action is needed.”
US mobile phone users will not be able to opt out of the test.
Former US president Barack Obama signed a law in 2016 which required FEMA to design a system where the president could send mobile phone alerts in national emergencies.
The president has responsibility for deciding when the nationwide alerts are used.
FEMA has issued more than 36,000 mobile phone alerts since launching a wireless emergency system in 2012.
The messages have varied from alerts about natural disasters and extreme weather to notifications about missing children.
Mobile phone users can opt out of receiving these standard alerts.
A presidential directive has never been issued through the system and FEMA said in a statement that the alerts could only be used for national emergencies.
The test could be rescheduled for 3 October if there is a major event on Thursday or if there is widespread severe weather, according to FEMA.
Parts of the US are currently in the grip of Hurricane Florence, which has killed at least eight people in the Carolinas.