HAYMARKET, Va. — FOUR years ago, the Federal Trade Commission announced, with fanfare, a plan to let American consumers decide whether to let companies track their online browsing and buying habits. The plan would let users opt out of the collection of data about their habits through a setting in their web browsers, without having to decide on a site-by-site basis.

The idea, known as “Do Not Track,” and modeled on the popular “Do Not Call” rule that protects consumers from unwanted telemarketing calls, is simple. But the details are anything but.

Although many digital advertising companies agreed to the idea in principle, the debate over the definition, scope and application of “Do Not Track” has been raging for several years.

Now, finally, an industry working group is expected to propose detailed rules governing how the privacy switch should work. The group includes experts but is dominated by Internet giants like Adobe, Apple, Facebook, Google and Yahoo. It is poised to recommend a carve-out that would effectively free them from honoring “Do Not Track” requests.