Keep calm and carry on: When the UK Emergency Alert system sounds nation-wide for the first time on Sunday (April 23) it will be a test.

For 10 seconds mobile phones and tablet devices across the country – that are 4G or 5G enabled – will make a loud siren-like sound and vibrate – or read out an alert.

The system has been been designed to alert residents in specific locations to emergencies that could cause a risk to life – such as severe flooding, fires or extreme weather.

And on Sunday – at 3pm – the system will be tested nation-wide for the first time.

Hertfordshire chief fire officer Alex Woodman says that during the test residents will hear a "very unique sound" from their phones – a repeated alert noise that runs for 10 seconds.

It will sound even if a phone is on 'do not disturb' or 'silent' mode – but won't work if the phone is turned off or on 'airplane' mode.

And Mr Woodman says it is an important, additional tool for emergency services

"My hope I that we never need to use it in Hertfordshire," said Mr Woodman, who is local resilience forums lead at the National Fire Chiefs Council.

"I wouldn't envisage a system like this being used regularly – maybe once or twice in a lifetime.

"When it happens on Sunday we don't want anybody to take any action at all. This is purely a test – and I don't envisage any panic."

Mr Woodman says that in future the alert system could be used, for example, to alert residents to the need to evacuate if they are in the path of a wild fire.

Fire and police officers would still be deployed, but – says Mr Woodman – residents could be alerted to the need to evacuate even before emergency services arrive at the scene.

And it could also be used to pass on vital information about where to evacuate to and safe routes to take – as well as areas to avoid.

Mr Wodman says the system has been in development for a number of years.

And is possible now because of 'cell broadcasting' that allows mobile phone operators to send messages on behalf of governments – at national or local level.

The technology is built-in to 4G and 5G phones, with no app to download, says Mr Woodman.

And, he says, the system can deliver alerts to 90 per cent of mobile phones within 10 seconds.

To send the same number of text messages, says Mr Woodman, could have taken a number of days.

Flooding, wildfires and extreme weather have already been earmarked nationally as possible uses, but Mr Woodman accepts that in future it could be considered for broader uses, such as terror attacks.

There have been concerns raised that the test, on Sunday, could reveal hidden or concealed phones.

Mr Woodman acknowledges people – including those living with domestic abuse – will have hidden phones.

And he says as much information has been put out as possible to show how to switch off the alerts.

While the alert WILL sound if a phone is on 'silent' mode or 'do not disturb', it will NOT sound if the device is tuned off or on 'airplane' mode.

Meanwhile Mr Woodman also acknowledges that like any other platform, fraudsters could try to exploit the alert system.

He stresses residents will never be asked to reply to an emergency alert – and that no emergency service provider would ever ask for personal incarnation such as passwords or pin numbers.

He urges anyone who is ever in doubt about a message they receive to contact the police or trading standards officers before they respond.

• The test of the Emergency Alert will take place at 3pm on Sunday (April 23).