Rep. Markey said, "Charter Communications' reported plans to sell information about their customers' activities online raises several red flags. Simply providing a method for users to opt-out of the program is not the same has asking users to affirmatively agree to participate in the program. These privacy issues and how this venture is consistent with communications privacy laws must be addressed before the company moves forward with this plan."

Charter Communications is the fourth-largest cable system in the United States. According to media reports the company is planning to start keeping track of every Web site consumers visit and then disclose the data to NebuAd, an online-ad firm.

Reps. Markey and Barton's letter refers to Section 631 of the Communications Act which contains privacy provisions regarding cable operators. The letter states that "Any service to which a subscriber does not affirmatively subscribe and that can result in the collection of information about the web-related habits and interests of a subscriber, or a subscriber's use of the operator's services, or the identification of an individual subscriber, and archives any of these results without the ‘prior written or electronic consent of the subscriber,' raises substantial questions related to Section 631."

The full letter is available here.