This is the moment people’s phones sounded for the national emergency test alert in the British Library.
At about 15:00 BST on Sunday, mobile phones connected to 4G and 5G networks vibrated and emitted a siren for up to 10 seconds.
It was the second test of the system after the first in 2023.
Mobile phone users also received a message making clear it was a drill.
The government has used the system to issue real warnings five times, including in January during Storm Eowyn to warn people in Scotland and Northern Ireland about severe weather.