Google has announced a new feature that allows users to share their locations among a chosen network of friends.
The "opt-in" Latitude service uses data from mobile phone masts, GPS, or wi-fi hardware to update a user's location automatically.
Users can also manually set their advertised location anywhere they like, or turn the broadcast off altogether.
Privacy advocates have raised the security concern that many users may not be aware that it is enabled.
Latitude is based on Google's My Location feature that has been in place since last year.
The new interface and social networking element makes Latitude similar to a number of websites such as Loopt and Brightkite that make use of the location data of a network of friends.
Users can set the service to update automatically using the best location data it can obtain from the phone's hardware, set the location to display at city level only, or to not send any location data at all.
Locations are shared only between people who mutually agree to share them, and users can also see their Latitude friends' locations on a computer.
Google says it has built the service from the ground up with security and privacy issues in mind, and that the service only stores the last known location of a given user.
However, privacy watchdog Privacy International argues that there are opportunities for abuse of the system for those who may not know that their phone is broadcasting its location.
Privacy International director Simon Davies gives the example of employers who might give phones to employees with Latitude enabled.
"With Latitude, Google has taken steps toward privacy that it has hitherto not taken," Mr Davies told BBC News.
"The problem is that they launched the services without allowing all phones to be notified."
Google admits that the notification service is currently only available for BlackBerry users.
"We have implemented a feature on the BlackBerry version of the software to display several notifications (i.e. pop-up messages) to a device which informs the user that his or her phone's location is being shared," said a Google spokesperson.
"We hope to extend this to other versions of the software soon," the spokesperson added, noting that all platforms should be supported within a week's time.
For Mr Davies, the issue is principally a philosophical one about the nature of privacy.
"I have absolutely no doubt, as a tech-lover, about the utility of this as extremely beneficial," he said.
"But it will be destroyed by privacy if the companies don't get it right."
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