MILLIONS of people are set to receive a nationwide 'emergency' pop-up on their mobile phones today - and this is why.

A number of mobile networks have said they are testing a flash-up alert between 1pm and 2pm on Tuesday.

It is understood it will mainly target iPhones, while a small number of Android devices will be included in the test alert.

For those receiving it, they may experience a loud siren-like sound before a pop-up confirming the test will appear.

The alert will say: "This is a mobile network operator test of the Emergency Alerts service. You do not need to take any action. To find out more, search for gov.uk/alerts."

Why is this happening

Millions of phones are receiving the alert ahead of the new service that is expected to launch in the UK this summer.

They are designed to alert people if there is a danger to life nearby, which could potentially be anything from terrorist incidents to severe weather.

According to the Government website, the phone or tablet will receive the alert with advice on how to remain safe.

Do they need my number?

The alert system uses information broadcast from mobile masts and sends messages to any devices within range.

That means the alert system does not need your mobile phone number, or you to even register, in order to work.

But for those who do not want the alert, you view how you can opt out by visiting the GOV.UK website here.

It is worth noting however, that you can only opt out of some emergency alerts - you cannot opt out from the "most important" ones.

A statement on the GOV.UK website explains: "You cannot opt out by subject, only by how serious the emergency is.

"If you opt out because you do not want flood warnings, for example, you might miss alerts for fires and terrorism.

"Because of this, you should keep emergency alerts switched on for your own safety."