The practice of dialing the operator on zero and asking for police had to be changed when five women lost their lives in a house fire in Wimpole Street in 1935.

Neighbours dialing zero found the operator exchange jammed, and there was no way to distinguish between life and death calls and others coming into the system.

A new way of working had to be introduced that let the operator exchange know it was an emergency.

After experiments the 999 number was settled on, and operators were alerted to incoming emergency calls with a series of red lights and back-up buzzers.

Saturday (July 1) marks the 80th anniversary of the 999 call being introduced in London, and much has changed in the intervening period.

The Metropolitan Police has published a detailed history of the life-saving service.

It revealed that the initial range of buzzers to alert operators of an emergency emitted such a loud foghorn-like noise that it distressed the switchboard staff to the point of fainting.

The Met Police says: "After discovering a tennis ball stuffed into the mouths of the buzzers muffled the noise to an appropriate extent, the General Post Office issued urgent instructions to all its local engineers to go out and buy tennis balls to fit to all the new buzzers!

The Lambeth Metropolitan Police contact centre

"Later on, adjustable covers were introduced to regulate the noise."

In the early days adverts were taken out in newspapers giving the public examples of "urgent" situations that constituted an emergency.

"Call 999 if for instance the man in the flat next to yours is murdering his wife, or you have seen a heavily masked cat burglar feeling round the local bank building," the adverts said.

Officers at work in a 1930s information room

"If you have lost little ‘Towser’, or a lorry has come to rest in your front garden, just call up the local police."

Back in 1937 the police received an average of 285 calls daily in London. The number of combined 999 and 101 calls made daily between January 2017 and May 2017 was 13,314.

At first a handful of police officers transmitted Morse code to wireless cars, now there are 1,550 staff at three high-tech centralised communications complexes in Bow, Hendon and Lambeth.

Chief Superintendent David Jackson, head of the Met Contact Centre, said: "The 999 system is the cornerstone not only of British policing but also for our emergency service partners.

1938 information room - officer has taken a call and gone to the map room

"The level of accessibility that it provides allows emergency services to save lives and protect the public. Our dedicated staff operate 24 hours a day, 365 days a year ensuring that police can be dispatched quickly to incidents where the public need us.

“Being there 24/7 and ensuring that officers are dispatched quickly to emergency and priority incidents is a critical function for the police. The progress since 1937 has been enormous - and we will keep striving to improve the service over the next 80 years."

The Metropolitan Police announced that it is recruiting for more call handlers to join the service.

The 1960s Scotland Yard telegraph office

Chief Superintendent Jackson added: “Eighty years is a significant milestone of the emergency service provision.

"The work is demanding, requiring a calm and collected analytical approach to the five million calls a year we now take.

"The work is exciting and no two shifts are ever the same and I’m hugely proud of the colleagues I represent who answer emergency and non-emergency calls from the public and dispatch our officers and staff to those incidents where the police are needed.”

For more information, visit the Metropolitan Police website here .

Keep up to date with the latest news in west London via the free getwestlondon app.

You can set up your app to see all the latest news and events from your area, plus receive push notifications for breaking news.

Available to download from the App Store or Google Play for Android .