A mobile phone service which allows users to 'track' their friends and family has been launched in the UK.

The Social Network Integrated Friend Finder (SNIFF), which has been created by US company Useful Networks, uses mobile phone signals to locate contacts and then displays their location to the nearest 100m on an electronic map sent via SMS. It employs similar technology, which is used by the police to identify the location of suspects or missing children through their mobile phone.

The service will be available only to those over 18 and users will need to register for SNIFF through social-networking sites Facebook, Bebo and MySpace, as well as online with Useful Networks itself. It will cost 50p per 'sniff'.

Friends and family must give permission to be 'tracked' and will also have the option to be made 'invisible' for as long as they want, ensuring the service doesn’t break any privacy laws.

The service, which is the first Facebook application to charge customers' mobile phone accounts, has more than 80,000 users in Sweden.

"This service is for people you have a trusted relationship with - people you give your car keys or the keys to your house," explained CEO Brian Levin.

"If you've ever found yourself stuck at home because your mates down the pub couldn't hear your calls above all the noise or missed out on an impromptu night out because you didn't realise that friends had been out and about near where you were, then sniff is for you," added Levin.