Google Inc. announced a major change to its powerful search engine to reduce the appearance of what it calls “low-quality” Web sites in results.

The move comes after months of criticism from a few technology-industry insiders and an acknowledgement by Google last month that it “can and should do better” to beat back sites that game its system to rise up in search results but offer users little value.

In a post on the company’s blog Thursday night, Google search engineers Matt Cutts and Amit Singhal said the change to Google’s search algorithm, which helps rank sites on the search results page based on how relevant they are to the user’s query, would lower the rankings for certain “low-quality sites” such as those that “copy content from other Web sites” or “sites that are just not very useful” but have found a way to rise up in Google’s rankings.

About 12 percent of U.S.-based search queries would be affected, they said.

“It has to be that some sites will go up and some will go down,” the Google engineers wrote, adding that sites with original content “such as research, in-depth reports, thoughtful analysis and so on” will move up.

Google didn’t give examples of Web pages that rose or dropped in its rankings for certain queries, setting off a wave of speculation by professionals whose job it is to help Web sites rise up in Google’s rankings, known as search-engine optimization, or SEO.

Many sites rely Web traffic from Google, and even a small drop in the rankings could potentially reduce revenue from posting ads on their pages.

One company that responded to Google’s move was Demand Media, a Web company that recently went public and runs large content sites such as eHow.com and Answerbag.com and relies on traffic from Google. It said in a blog post Thursday that “we haven’t seen a material net impact” from the changes, though its shares traded lower on the news.

Danny Sullivan, who writes an influential blog called Search Engine Land, said that an eHow page with “shallow” content recently appeared as the first Google search result when users typed “how to get pregnant fast” into the search box. Since Google’s change, the eHow page has dropped out of the top search results.

Google said the effort to beat back low-quality sites has been under way for about a year and that changes will roll out to non-U.S. users soon. Google handles about two-thirds of search queries globally, according to comScore Inc.

To learn what sites users find to be of poor quality, Google this month began offering what’s known as an extension for the Google Chrome browser. The experimental extension, which Chrome users can download, allows them to manually block sites from their search results if they deem them to be spam or low-quality. Once blocked, the sites won’t appear in future searches.

Google said Thursday that though it didn’t use data from the experiment to influence the algorithm changes, it found that the change covered 84 percent of the Internet sites that were the “most-blocked” by users.

One new competitor to Google, startup search engine Blekko, relies on its users to weed out what they believe are poor sites in various categories such as health, cars and personal finance.

Web companies work hard improve their search ranking, but sometimes they use tactics that are against Google’s stated rules. This week Google penalized Overstock.com for alleged violations, pushing the site down in its rankings for many searchers. Overstock cited the way it had solicited Web sites of universities and colleges to include links to Overstock on their sites, in exchange for offering students and faculty a 10 percent discount on Overstock goods.

The debate over Google’s search results hasn’t affected its commanding lead in terms of search market share, despite progress by Microsoft Corp.’s Bing search engine.