required by law to be compatible with the SAE system since 2017. United Kingdom Between 1953 and 1992, the UK government had an alert system known as the "four-minute warning". Its purpose was to warn the population, through a combination of air-raid sirens and messages over television and radio, of an impending nuclear missile attack from the Soviet Union; its name derived from the expected amount of time between knowledge of the strike and the missile landing. The system was dismantled following the dissolution of the Soviet Union and the end of the Cold War. In 2013, the government conducted trials of a public alert system using both SMS messaging and Cell Broadcast technology to send alert messages to mobile devices in areas affected by an emergency. A report was published in 2014 that said the trials were successful and that 85% of people thought that such a system would be a good idea, as well as detailing how such a system could be implemented, and plans for further trials. However, no further trials or implementation of the system came. On 24 March 2020, the day after the UK went into a national lockdown in response to the COVID-19 pandemic, the government collaborated with the country's four main mobile networks, O2, EE, Vodafone and Three, to send a text message alert regarding the new restrictions to all registered mobile phone numbers across the UK. In order to not overwhelm the networks, the operators sent the messages in batches, going to one million or so numbers at a time. This process took over 24 hours, with some people not receiving the message until the following day. The government was accused of ignoring its own advice from the 2013 trials to set up a dedicated public alert system. In response, it began developing a system using Cell Broadcast technology. In March 2021, a test flood alert was sent using the new technology to devices in Reading. Another generic test alert was sent to O2 customers on 11 May. Certain BT Mobile customers (with BT Mobile operating on the EE network) received two alerts on 20 May. A more public test of the system was carried out for devices in East Suffolk on 25 May, followed by a test across the UK on 12 June. Further limited tests were sent on 18 and 22 June. All of these test messages were sent specifically