Correction: An earlier version of this article did not make clear that users who have not logged on to Google or one of its other sites, such as YouTube, are not affected by the new Google privacy policy. This version has been updated to include that information.

Google will soon know far more about who you are and what you do on the Web.

The Web giant announced Tuesday that it plans to follow the activities of users across nearly all of its ubiquitous sites, including YouTube, Gmail and its leading search engine.

Google has already been collecting some of this information. But for the first time, it is combining data across its Web sites to stitch together a fuller portrait of users.

Consumers won’t be able to opt out of the changes, which take effect March 1. And experts say the policy shift will invite greater scrutiny from federal regulators of the company’s privacy and competitive practices.

The move will help Google better tailor its ads to people’s tastes. If someone watches an NBA clip online and lives in Washington, the firm could advertise Washington Wizards tickets in that person’s Gmail account.

Consumers could also benefit, the company said. When someone is searching for the word “jaguar,” Google would have a better idea of whether the person was interested in the animal or the car. Or the firm might suggest e-mailing contacts in New York when it learns you are planning a trip there.

But consumer advocates say the new policy might upset people who never expected their information would be shared across so many different Web sites.

A user signing up for Gmail, for instance, might never have imagined that the content of his or her messages could affect the experience on seemingly unrelated Web sites such as YouTube.

“Google’s new privacy announcement is frustrating and a little frightening,” said Common Sense Media chief executive James Steyer. “Even if the company believes that tracking users across all platforms improves their services, consumers should still have the option to opt out — especially the kids and teens who are avid users of YouTube, Gmail and Google Search.”

Google can collect information about users when they activate an Android mobile phone, sign into their accounts online or enter search terms. It can also store cookies on people’s computers to see which Web sites they visit or use its popular maps program to estimate their location. However, users who have not logged on to Google or one of its other sites, such as YouTube, are not affected by the new policy.

The change to its privacy policies come as Google is facing stiff competition for the fickle attention of Web surfers. It recently disappointed investors for the first time in several quarters, failing last week to meet earnings predictions. Apple, in contrast, reported record earnings Tuesday that blew past even the most optimistic expectations.

Some analysts said Google’s move is aimed squarely at Apple and Facebook — which have been successful in building unified ecosystems of products that capture people’s attention. Google, in contrast, has adopted a more scattered approach, but an executive said in an interview that the company wants to create a much more seamless environment across its various offerings.

“If you’re signed in, we may combine information you’ve provided from one service with information from other services,” Alma Whitten, Google’s director of privacy for product and engineering, wrote in a blog post.

“In short, we’ll treat you as a single user across all our products, which will mean a simpler, more intuitive Google experience,” she said.

Google said it would notify its hundreds of millions of users of the change through an e-mail and a message on its Web sites. It will apply to all of its services except for Google Wallet, the Chrome browser and Google Books.

The company said the change would simplify the company’s privacy policy — a move that regulators encouraged.

Still, some consumer advocates and lawmakers remained skeptical.

“There is no way anyone expected this,” said Jeffrey Chester, executive director of the Center for Digital Democracy, a privacy advocacy group. “There is no way a user can comprehend the implication of Google collecting across platforms for information about your health, political opinions and financial concerns.”

Added Rep. Edward J. Markey (D-Mass), co-chair of the Congressional Privacy Caucus: “It is imperative that users will be able to decide whether they want their information shared across the spectrum of Google’s offerings.”

Google has increasingly been a focus of Washington regulators.

The company recently settled a privacy complaint by the Federal Trade Commission after it allowed users of its now-defunct social-networking tool Google Buzz to see contacts lists from its e-mail program.

And a previous decision to use its social network data in search results has been included in a broad FTC investigation, according to a person familiar with the matter who spoke on the condition of anonymity because the investigation is private.

Federal officials are also looking at whether Google is running afoul of antitrust rules by using its dominance in online searches to favor its other business lines.

Claudia Farrell, a spokeswoman for the FTC, declined to comment on any interaction between Google and regulators on its new privacy changes.