On Thursday, a House subcommittee will hold a hearing exploring whether Congress should require such an option.
The government has helped millions of Americans avoid annoying phone calls from telemarketers. Now, momentum is growing in Washington to give consumers the same power to block companies from tracking their every movement on the Internet.
Building off the popular do-not-call registry, the new effort would create a do-not-track mechanism to prevent companies from monitoring which websites people search for and visit.
The website tracking gives companies the coveted capability of targeting ads based on a person's behavior, but it is increasingly viewed as a troubling invasion of privacy.
"There are no limits to what types of information can be collected, how long it can be retained, with whom it can be shared and how it can be used," said Susan Grant, director of consumer protection for the Consumer Federation of America. "Consumers simply have no legal control over being spied on when they go online."
On Wednesday, the Federal Trade Commission endorsed a do-not-track mechanism, though it is asking for voluntary industry participation. And on Thursday, a House subcommittee will hold a hearing exploring whether Congress should require such an option.
The initiative faces opposition from many in the online industry, who say it could be technically difficult to implement — and could put the brakes on booming Internet commerce.
"If a broad percentage of people signed on to this, it would really undercut the Internet model," said Stuart Ingis, counsel to the Digital Advertising Alliance, a trade group whose members include Google Inc., Microsoft Corp. and Yahoo Inc.
The Obama administration's position is unclear, but online privacy legislation is expected to be introduced by lawmakers early next year and could include a do-not-track provision.
The issue could be one of the few areas of bipartisan agreement in a divided government. Many Democrats and Republicans support increased protections for Americans' privacy, particularly after a series of high-profile incidents involving inappropriate data collection.
Among those incidents is the recent disclosure that Google, in driving through neighborhoods snapping photos for its Street View mapping feature, also collected e-mails, passwords and other information from unsecured home Wi-Fi networks.
Google said the data collection was inadvertent, but the Federal Communications Commission is investigating.
"Industry efforts to address privacy through self-regulation have been too slow and, up to now, have failed to provide adequate and meaningful protection," the FTC said in a long-awaited privacy report released Wednesday.
Websites offer detailed privacy policies about how they might use any data collected, and some ad companies allow people to opt out of their invisible tracking efforts. Just last week, a broad coalition of Internet companies launched an initiative to allow consumers to block much of the online data collection.
But privacy advocates said those efforts have fallen short. They argue that consumers are in a technological arms race with Internet companies in which new tools to stymie data collection are overcome by new methods for obtaining it.
"The trackers are always two or three steps ahead of any privacy-enhancing technologies that people can realistically use," said Peter Eckersley, a senior staff technologist at the Electronic Frontier Foundation, a digital rights group.
He and other privacy advocates said the answer is a do-not-track option that advertisers would be required to honor, just as telemarketers must not call people who place their phone numbers on the do-not-call list. That list, begun in 2003, now contains about 200 million phone numbers.
Three years ago, a coalition of privacy and consumer groups proposed creating a similar list for people who did not want their Internet activity tracked. But consensus now is coalescing around a do-not-track option built into Web browsers.
The mechanism, which consumers would have to activate once, would send a signal to each website visited indicating that the person's data should not be tracked and that the person did not want to receive advertisements targeted to past searches or other online history.
FTC Chairman Jon Leibowitz said many consumers might decide they do not want to opt out of data tracking, which can deliver advertisements targeted to a person's interests.
But a poll done this summer for Consumer Watchdog, a California public interest group, found that 84% of respondents wanted to prevent online companies from tracking personal information without a person's explicit, written approval.
Microsoft, Mozilla and Google, which make the most popular Web browsers, are exploring the feasibility of a do-not-track option, Leibowitz said.
"At this point we would just like to see the browser vendors move a little more quickly, and it could be done voluntarily by companies to give consumers the protection we envision them having," he said.
Google said that the commission's report raised "some interesting ideas, and we look forward to learning more about what do-not-track could look like."
Google and other companies have stressed privacy protections already built into their products.
The Digital Advertising Alliance last week launched a long-awaited industry initiative called the Self-Regulatory Program for Online Behavioral Advertising, at http://www.aboutads.info. It allows people to check their Web browser for tracking devices known as cookies placed by participating companies and then opt out of receiving targeted ads.
Consumers soon will start seeing a new "advertising option icon" on advertisements that links to the site. The program isn't as broad as a do-not-track mechanism built into the Web browser.
Leibowitz and privacy groups have criticized the industry for taking two years to launch the initiative.
Ingis said the criticism is fair, but developing the technology and the program has been complex. A federal mandate would be even more difficult.
"The notion that the government would be mandating technology, we just don't think they should do that," he said.
The Commerce Department is working on its own privacy report to be released soon. The report is expected to call for a new government privacy office to organize voluntary, industrywide efforts that would address issues faster than regulators or Congress.
