Apple iPhones and 3G iPads are secretly recording and storing details of all their owners' movements, researchers claim.
Location data is kept in a hidden, unencrypted file according to security experts Alasdair Allan and Pete Warden.
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 there is no suggestion that it has been uploading or using the information.
The findings, first reported by the Guardian newspaper, will come as a surprise to most iPhone users, as their devices do not give any visual indication that such data is being recorded.
"We may collect information such as occupation, language, zip code, area code, unique device identifier, location, and the time zone where an Apple product is used so that we can better understand customer behaviour and improve our products, services, and advertising"
Writing on the technology website O'Reilly Radar, Mr Allan and Mr Warden said they did not know why iPhones and iPads were collecting location information but it was "clearly intentional".
The men claim that the facility to record users' positions was added with the iOS4 software update, released in June 2010.
The data is also transferred to the owner's computer and stored in a file there each time the two devices are connected to carry out a back-up or synchronisation.
Graham Cluley, senior technology consultant at security firm Sophos, told BBC News that it was unlikely Apple planned to use the information for commercial purposes.
"I think there are some legitimate privacy concerns and people will probably look for a way of obscuring that data," he said.
"But it is an object lesson about reading the terms and conditions," he added.