Ahead of the 2023 test, domestic abuse charities warned that the alert system could potentially endanger victims by alerting an abuser to the existence of a secret phone.

The National Centre for Domestic Violence advised people with concealed devices to make sure they were turned off for the duration of the test.

The government has stressed that emergency alerts should remain switched on, but has published a guide, external for domestic abuse victims on how to opt out.

• What to do about the emergency alert if you’re a victim of domestic abuse
• BBC Action line: Information and support about domestic abuse

The first nationwide test of the emergency alert system was conducted in April 2023.

That test revealed a number of technical issues - with some people getting the alert more than once and others not receiving it at all.

Some users also got the alert ahead of the scheduled time while others got it later.

Many mobiles phones on the Three network did not get anything, along with some users on other networks. Others received multiple messages.

The government later confirmed that the alert had not reached around 7% of compatible devices, including those which were turned off or on aeroplane mode, and some where the user had opted out of emergency alerts.

As a result, the government promised to address the technical issues.

Today's emergency alert will be sent to all smartphones on the UK's 4G and 5G networks, even if they are not connected to mobile data or Wi-fi.

There are about 87 million mobile phones in the UK and the government says about 95% of the population has 4G or 5G access.

Older phones, and phones connected to 2G or 3G networks, will not receive the message.

Phones that are switched off or in flight mode and those that opt out will also not get the alert.

The government strongly recommends that users enable devices to receive alerts, but it is possible to opt out.

This can be done by searching in your phone's settings for "emergency alerts" and turning off "severe alerts" and "extreme alerts".

The government has also published a guide on how to opt out of the test., external

Image source, Getty Images

The national system for sending emergency alerts to mobile phones is being tested for the second time today at 15:00 BST.

The alert will be sent to millions of smartphones on the UK's 4G and 5G networks. The devices will vibrate and make a siren sound for roughly 10 seconds.

All compatible phones - the vast majority of those currently in use - are expected to get the alert.

The alerts are designed for situations where there is an imminent danger to life, such as during extreme weather events or a terror attack.

The previous test, in April 2023, revealed a number of technical issues - with some people getting the alert more than once and others not receiving it at all.

We'll be following the test and bring you the results and reaction on this page - so stick with us.