“There is vast consumer awareness and concern about privacy,” says Fatemeh Khatibloo, a senior analyst in customer intelligence at Forrester Research. “If a browser can differentiate itself by saying ‘we provide you better privacy tools,’ I think they’ll increase adoptions.”

But the specter of people opting out of tracking en masse presents a serious risk for marketers.

Consumer data, marketers say, is the fuel that powers the Internet, driving ads that support free content and e-mail services, search engines and social networks. If millions of consumers opted out of behavior-based advertising, industry representatives argue, many ad-sponsored sites could shut down or put up pay walls for people who elect not to see the ads. Internet Explorer 10 is only heightening their concerns. Because consumers tend not to change preset technology options, advertisers worry that the browser could shift millions of people to the do-not-track category.

“That would drastically skew the economic model underlying the Internet,” says Stuart Ingis, counsel to the Digital Advertising Alliance. “The choice is the Internet as we know it, or a much smaller, cannibalized Internet where you don’t have the diversity.”

Developers at Apache, a popular Web server, have also objected, saying that Microsoft’s default setting may not convey a user’s specific intent. They have introduced an update that may cause some sites to ignore what they view as a “presumptive” do-not-track flag.

Erica Harbison, a spokeswoman for Microsoft, said company executives were unavailable to comment for this column. She referred me to company blog posts on the topic.

At the moment, however, advertisers’ concerns are mainly theoretical.

The do-not-track browser option is still an emerging concept. Last year, an international standards body called the W3C, or World Wide Web Consortium, created a working group to standardize the technology for do-not-track systems. But marketers and privacy advocates are at odds over even the definition of “do not track.” Does it mean “do not show personalized ads to a particular user”? Or does it mean “do not collect information about a person’s browsing history”? There is also disagreement over how sites should acknowledge and respond to the signal.

Still, the likelihood of millions of consumers choosing that option has provoked a fierce debate.

Industry representatives say privacy advocates have skewed the conversation from the outset, by using Big Brother-y terms like “do not track,” when they view the choice for consumers as between seeing relevant ads or generic ads. They add that the process shouldn’t scare consumers. To create interest-based ads, they say, ad networks and analytics companies assign people anonymous code numbers and simply record things like the sites they visit and the search terms they enter.