THE Government has launched a probe into why millions of Brits did not receive an emergency alert during yesterday's test.
The Cabinet Office will lead a review into the issue after it caused confusion in households across the UK.
A Government spokesperson said: "We have effectively completed the test of the UK-wide Emergency Alerts system, the biggest public communications exercise of its kind ever done.
"We are working with mobile network operators to review the outcome and any lessons learned."
At 3pm on Sunday millions of Brits were met with a loud siren noise coming from their phones.
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The piercing 10-second sound was accompanied by a message confirming a test of the Government's emergency alert service was being run.
The system will be used in future to inform Brits when their lives are in danger.
While many received the texts, millions also missed out - mostly because their service provider was Three.
A spokesperson for the network said: "We are aware that a number of customers have not received the test alert.
"We are working closely with the Government to understand why and ensure it doesn't happen when the system is in use."
Deputy PM Oliver Dowden insisted yesterday that the alert system could save lives in the future.
Ahead of the test he said: "Our number one job is to keep people safe and this is another tool in the toolkit for emergency situations, such as flooding or wildfires, and where there is a genuine risk to life.
"I would encourage people to remember that today it is just a test. There is no need to take any action and you can simply swipe it away as you would any other message you receive."
The test caused a number of surprises for those who forgot it was coming - or weren't aware it was about to happen.
Some London marathon runners were caught off guard by the loud noise as they approached the finish line.
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And a BBC news presenter tried not laugh as his phone went off during an interview about the test.
The Cabinet Office yesterday confirmed that the "vast majority of compatible phones" received the alert.