People will soon be able to find their friends using their mobile phone signals.

The Social Network Integrated Friend Finder (Sniff) has been developed by Useful Networks. It will be accessible through Facebook and can be used by those on the T-Mobile, Orange, Vodafone and O2 networks.

It works through similar technology used by the police to track down suspects or missing children via their mobile phone. The phone sends a signal to nearby base stations and software performs a calculation on the information from the base stations to convert it into a geographical location.

People over 18 on Facebook can ask their friends to sign up to the application and give their permission to be tracked. The age is also verified by network providers. The service costs 50p per 'sniff', with the location of a friend being sent by text. However, Useful Networks said the service should not be relied upon by parents to track their children, because the service will only place a location within a radius of several hundred metres.

It also said people who had signed up for the service could choose whether to be visible or not.

Brian Levin, chief executive at Useful Networks, said: “You are fully in control of who can track you and when; you can go invisible any time – that is paramount.

“This service is for people you have a trusted relationship with; people you give your car keys or the keys to your house. The most asked question on mobile phones is 'where are you?’ and there are a lot of cases where you don’t want to answer your phone because it is too loud or have the hassle of texting.”

However, Michael Holloway, a representative for privacy organisation the Open Rights Group, warned that the technology could be dangerous.

He said: "The technology to track people via their phones has been available for quite some time, but as with all systems, the devil is in the detail, of which we are unaware. One interesting and useful implication of Sniff would be addressing the concept of 'friend' in the context of social networks, which has been until now largely meaningless.”

Facebook was not available for comment.