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.