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By Paul Seddon
Politics reporter
Sunday's emergency alert did not reach one in five compatible smartphones in the UK, the government has said.
It marked the first nationwide test of a new system to warn people about dangerous situations, such as floods or terror attacks.
The Cabinet Office estimated 80% of eligible phones received it, which they said was a "successful test".
Officials said they were working with Three UK, some of whose customers did not get the alert.
The Cabinet Office said there were "no current plans" for a further UK-wide test, but there are likely to be further public tests of the system in the coming years.
The alert included a short message, accompanied by a loud ten-second noise and vibration. Smartphones that were turned off or on aeroplane mode did not receive it.
In a statement to Parliament, deputy PM and Cabinet Office minister Oliver Dowden said Sunday's test was a "critical step forward" in the UK's ability to respond to emergencies.
He added that it authorised the alert just after 14.59 BST, in order to "minimise disruption" to events scheduled to start at 15.00 BST.
"As intended, the broadcast continued until 15:21, when the Cabinet Office issued the instruction for networks to stop transmitting," he said.
"One mobile network provider, Three UK, experienced an issue with supporting multiple messages. This led to some Three customers failing to receive the emergency alert".
He added that the Cabinet Office was working closely with Three to "implement an appropriate fix to ensure that this does not happen for them with future emergency alerts".
On Monday, the company said its engineers had fixed a "technical issue" and that "there will be no issue with future alerts".
'Autocorrect error'
Mr Dowden also blamed a "small autocorrect" for a Welsh language error included in the bilingual Welsh-English version of the alert sent to people in Wales.
For the words "others safe", the message read "eraill yn Vogel" when it should have been "eraill yn ddiogel".
He added that people travelling between England and Wales during the test would have received two alerts, and this would be "addressed as part of the lessons learned exercise".
Sunday's alert was sent to 4G and 5G phone networks, on iPhones running iOS 14.5 or later or phones and tablets running Android 11 or later.
Before the test, the government estimated this meant nearly 90% of mobile phones in the UK would be eligible to receive it.
Many countries around the world use emergency-alert systems, including the United States, the Netherlands and Japan.