Every American is to receive a phone message from Donald Trump testing an emergency alert system to be used in the event of a terror attack.

For the first time a Presidential alert will be sent out to all American phones on Wednesday to test a programme which allows the authorities to warn them of national-level threats including natural disasters and terrorism.

The Wireless Emergency Alert involves a tone, which the agency admits could be described as “quite loud”, and a vibration, which is played twice, accompanied by a pop-up message which reads “THIS IS A TEST of the National Wireless Emergency Alert System. No action is needed.”

Phones switched to silent will just receive the visual message, which will have the headline “Presidential Alert”. The text message will be sent to phones within a half hour period, beginning at 2.18pm EDT, with the TV and radio messages broadcast two minutes later, at 2.20pm [7.20pm BST].

The system will affect users with a US phone number, as well as some visitors, the agency said, and it is not possible to opt out, though some older phones do not support the technology.

The system is also used for lower-level alerts such as local weather warnings and AMBER Alerts, which are used to alert the public to missing children. Unlike the presidential message, these can be switched off.