Kill switches that render stolen smartphones useless were credited on Wednesday with stifling robberies and thefts in London, New York City, and San Francisco.

In London, the number of reports of smartphones being stolen from people dropped 40 percent last year, after kill switches were introduced there, according to officials.

San Francisco last year recorded a 27 percent decrease in overall mobile phone robberies, and a 40 percent plunge in robberies targeting iPhones, according to the district attorney here.

California-based Apple was the first company to build in kill switches that let people remotely disable its smartphones, making them worthless to thieves interested in selling them to new users.

In New York City, there was an overall decline of 16 percent in mobile phone robberies and a 25 percent drop in iPhone robberies, officials there reported.

In London, the monthly average for the number of phones stolen has halved since September 2013 resulting in 20,000 fewer victims annually, according to figures released by mayor Boris Johnson.

"The significant decrease in smartphone thefts since the implementation of kill-switch technology is no coincidence," New York Police Commissioner William Bratton said in a joint release.

"Restricting the marketability of stolen cell phones and electronic devices has a direct correlation to a reduction of associated crimes and violence, as evidenced in London, San Francisco and New York."

New York attorney general Eric Schneiderman, San Francisco district attorney George Gascon, and Johnson co-chair a Secure Our Smartphones initiative launched in June of 2013 to find solutions to a violent crime wave of smartphone thefts.

The initiative pressed the smartphone industry to adopt kill switch technology as a theft deterrent.

Apple's home state of California was the first in the nation to mandate kill switches in smartphones, passing a law that will take effect in July of this year.

Apple added the technology to iPhones in September of 2013 in the form of an Activation Lock feature added to the iOS 7 version of its mobile software and made it standard in new-generation iPhone 6 models.

Activation Lock calls for an Apple ID and password to reactivate an iPhone that has been remotely disabled by its owner.

South Korean consumer electronics titan Samsung released a "kill switch-type solution" last year for its Galaxy S5 smartphone, officials noted.

Google added a smartphone disabling feature to the Lollipop version of its Android mobile device software, and Microsoft is expected to have the capability built into a version of its smartphone operating system due out later this year.

"The wireless industry continues to roll out sophisticated new features, but preventing their own customers from being the target of a violent crime is the coolest technology they can bring to market," Gascon said.