Mobile devices will sound and vibrate for up to 10 seconds when a nationwide test of the UK Emergency Alert scheme is activated for the first time this month.

The exact hour has been chosen to avoid clashes with football matches and the London Marathon when it is tested on the afternoon of St George's Day, Sunday, April 23.

As well as the audible alert and vibration, users will see a message on the home screen of their device and will need to tap continue or swipe it away to remove the alert, in much the same way as a low battery notification.

The so-called Armageddon alarm will sound at 3pm on April 23 with the Government working with emergency services, the Football Association and the London Marathon to make sure the test has minimum impact to major events taking place on the day.

In the Premier League two games kick off at 2pm, meaning the alarm will sound just as the second half gets under way as Bournemouth host West Ham and Newcastle play Tottenham.

In the FA Cup, Brighton v Manchester United kicks off at 4.30pm, but fans travelling to the match will receive the test. Anyone who is driving, should ignore the alert in the same way they would a text message.

The mass race of the London Marathon, which starts from 9.30am, has an eight hour time limit. Anyone running with their phone, and hoping for a time of around five and half hours, will see, hear and feel the alert as they cross the finish line.

Once the system is live, officials say it will be deployed to warn people in the event of emergencies such as wildfires or floods.

The April 23 test is not the first trial of the system. Localised broadcasts have previously taken place in East Suffolk and in Reading and the same technology is used around the world including the US, Canada and the Netherlands.

In Japan, it has been widely credited with saving lives during severe weather events.

For the test to work, a device needs to be switched on and emergency alerts setting must be activated. They are usually on by default, so anyone who wants to opt out would have to find emergency alerts in their phones settings and switch them off.

Once up and running the technology will allow an estimated 90 per cent of mobile phones in a defined area to receive the crucial updates.

But the system will be used rarely, says the Government, and only when there is an immediate risk to people's lives.

MP Oliver Dowden, Chancellor of the Duchy of Lanaster, said: "Getting this system operational with the national test means we have another tool in our toolkit to keep te public safe in life-threatening emergencies It could be the sound that saves your life."

Mark Hardingham, chair of The National Fire Chiefs Council, said: "We must use every tool at our disposal to keep people safe and we need everyone to play their part - and the new Emergency Alerts system is one way we can do this. For 10 seconds, the national test may be inconvenient for some, but please forgive us for the intrusion, because the next time you hear it - your life, and the life-saving actions of our emergency services, could depend on it."

Assistant chief constable Owen Weatherill, the National Police Chiefs' Council lead for civil contingencies, said: "Warning and informing the public at speed during times of crises can be vital. We look forward to further developing the use of the Emergency Alerts capability and how it can have real benefits for the public to protect and preserve life as well as supporting policing's wider response to critical incidents with partner agencies."

The Government has also been working with organisations that support vulnerable women and girls to ensure they are not adversely affected by the introduction of Emergency Alerts, because a concealed phone is revealed.

To opt out, simply turn the phone off or place into flight mode, or alternatively, find 'emergency alerts' in setting and turn off "severe alerts" and "extreme alerts".

The Government hopes that most people keep them turned on, to make the system as useful as possible in the face of a real threat.