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Louise Haigh signed off on a nationwide emergency wildfire alert that startled millions of people across the country.

Ms Haigh, who is head of the Cabinet Office, is understood to have approved the alert, which led to panicked Britons jamming 999 lines and startled drivers swerving across motorways.

Sirens blared out unexpectedly on millions of mobile phones across the country at 7pm on Friday, warning the public against lighting anything that could cause a wildfire, such as barbecues, firepits and fireworks.

It was the most extensive use of the UK's emergency alert system to date.

The Conservatives, who devised the system more than a decade ago, said the alert was only intended to be used in cases where there was an imminent, direct and mainly localised threat to life.

A Cabinet Office report explicitly warned against its use for 'lower level' incidents for fear of creating a 'cry wolf' effect, in which phone users either disabled or ignored the alert in cases of genuine emergency.

In a press release published shortly before the alert, Ms Haigh said: 'In response to the continued risk posed by wildfires, the Government has taken the decision to issue an Emergency Alert to England and Wales.

'This is the most extensive use of the UK's Emergency Alert system in response to an active incident, and reflects the seriousness with which the Government is acting to safeguard people and property.'

Louise Haigh signed off on a nationwide emergency alert that startled millions of people across the country

Millions of people received this alert last night as people were warned not to light anything that could spark a fire

The government had initially considered sending a targeted alert to the areas which were most affected or at risk of wildfires.

However, by the afternoon, Ms Haigh's department decided to release the alert across the country following calls from the Fire and Rescue Service, The Telegraph reported.

A Conservative source told the Daily Mail yesterday: 'It was designed for terrorist attacks, not to tell people how to cook their sausages.'

Frustrated cinema-goers and broadcasters added to the complaints about the disruption, while there was confusion across Wales after the alert was sent out in Welsh – despite only a quarter of the country speaking it.

Some residents received the message four times – twice in Welsh and twice in English.