A new mobile phone service, which lets users track their friends and family using their mobile has launched in the UK.

The Sniff application, from US firm Useful Networks, sends users a message showing the position of the person they are trying to find on a map.

But the other person has to have signed up to the service, and users always have the option of going "invisible" (so when you are going to the pub, for example, instead of to the gym as promised).

"The key thing is that there is an explicit opt-in, so you have to fully accept that you allow yourself to be located", the company explains.

"We say it's like deciding who you would give the keys to your car, or who you'd ask to watch your flat while you're on holiday."

The service works across all UK mobile networks, and has already won 100,000 users in Scandinavia.

Initially, the service charges users 50p a time, but may eventually become free and ad-supported.

Phone owners can sign up by registering their mobile by texting 60506 but can also add a Sniff application to their Facebook profile.

The service only accepts users who can verify they are over 18 through their mobile phone contracts.