The platform, which the telecoms giant plans to roll out next year, would lay out all the data that it has in its possession through its clients' phone usage and which Internet groups currently have access to.
The goal is to "let clients know what data internet firms have, so the client can decide what to do with them" -- which information he or she wants to share, and which they do not -- the spokesman, who refused to be named, told AFP.
If the client does not want to share the data, Telefonica, which is amongst the five biggest telecoms firms in the world, could block the information.
The spokesman said this could pave the way for big Internet firms to offer to pay for the data if the customer is willing to share it.
The plan, presented by Telefonica chairman Jose Maria Alvarez-Pallete on Monday in the northern city of Santander, targets firms like Facebook and Google which profit by selling personal data to marketers.
The market for such personal consumer data is worth billions of dollars and its use by internet firms has come under fire from privacy watchdogs and consumers.
Such is the use of personal data that Facebook, for instance, can work out roughly how much a user earns from which telephone operator and operating system he or she has, and the resolution of the mobile phone's screen.
Fight for data control
Last month popular messaging service WhatsApp said it would start sharing users' data with parent Facebook, a move which will allow for more relevant advertisements on the social networking site.
The announcement sparked a flurry of articles in the press and online explaining how WhatsApp users could opt out of sharing account information with Facebook, underscoring concerns over privacy.
Phone operators have long complained that these big US companies use the infrastructure belonging to Orange, Vodafone, Telefonica or others to transfer an increasing flow of data without having to pay to broaden or update the system.
Ivan San Felix, an analyst at brokers Renta 4, said that these operators "know that it hurts these companies to limit or reduce their capacity to obtain user data, and it looks like they are focusing on this issue."
Telefonica, which provides mobile and fixed communication services primarily in the European Union and Latin America, said it did not want to charge Google, Facebook and others for the data.
But "if Telefonica and other operators can control data more than before, this gives them more power to negotiate with these companies," San Felix said.
Anais Perez, head of communications for Google Spain, countered that the US company has for two years offered its users such a platform.
Called My Account, users can look up their data and manage it, as well as see what is being done with the information, erase searches, block ads and even erase their account.
"Google users already know about their data," she said.
By Alvaro Villalobos