The Social Network Integrated Friend Finder (Sniff) uses mobile phone signals to locate friends and partners, regardless of where they are in the UK.

The application will be accessible through the Facebook social networking website and across all the major mobile phone networks.

Useful Networks, the US company behind the technology, said the application would only be available to over-18s and those who gave their permission to be tracked.

Friends had control over who could find them and when, and could instantly switch between being "visible" and "invisible".

Mobile customers will pay 50p for each "sniff" and will receive the answer by text.

It will be the first Facebook application to apply premium charges to customers' mobile bills.

The application is popular in Sweden, where more than 80,000 people are registered to use it.

"Sniffing" 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 and converts it into a geographical location.

Brian Levin, chief executive of Useful Networks, said: "People are spending increasing amounts of time managing and mapping their friendship groups online, so why not literally map them offline too.

"Sniff is for people to use within their close friendship circles. It's easy, fun and it works. And because you have absolute control over if, how and when your location information is shared, it is also very safe too."

Mr Levin cautioned that sniffing should not be relied upon by parents to track their young children because the service would only place a location within a radius of several hundred metres.

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