In a special report published on Friday, The Wall Street Journal said Google, working together with numerous advertising agencies, placed a special computer code on millions of iPhones that permits the companies to track user behavior.
Google has denied that it monitors the activities of iPhone users and said the imbedded code, or cookies, tracks users and is only made active when users opt-in to one of Google's services, for example Gmail.
However, the company admitted that the code unintentionally allowed additional Google Web advertising cookies to be installed on users' phones without their permission.
Apple's Safari web browser obstructs tracking behavior, but Google's code "tricks" the Safari browser into monitoring iPhone user behavior, the report said.
Stanford University researcher Jonathan Mayer recently discovered the code. Acting on Mayer's advice, a technical advisor to the The Wall Street Journal discovered that the Google tracking code was installed on iPhones by 23 of the top 100 websites.
The code was implanted on advertisements from Fandango.com, Match.com, AOL.com, TMZ.com, UrbanDictionary.com, and a number of other sites.
After the code is installed, Google can follow user movement across a large number of websites.