Apple iPhones and some iPad tablet computers are secretly recording and storing details of their users' movements.
British security experts say they have discovered that the data is kept in a hidden unencrypted file, which is automatically transferred to any computer synchronised with the devices, the BBC reports.
With the right software, it can be used to map exactly where a person has been.
Apple has yet to comment on the revelation, however, the BBC says there is no suggestion that it has been uploading or using the information.
The technology experts cited by the BBC, Alasdair Allan and Pete Warden, said they did not know why iPhones and iPads were collecting location information but it was "clearly intentional".
They claim that the facility to record users' positions was added with the iOS4 software update, released in June 2010.