Online search engine leader Google Inc. added 1 billion pages to its Web index Tuesday, increasing its breadth by about one-third as it girds for tougher competition from Yahoo! Inc. and Microsoft Corp.

Google's search engine now spans 4.28 billion Web pages, up from 3.3 billion pages earlier this week. The Mountain View, Calif.-based company also said it has enlarged its index of Web images to 880 million, up from slightly more than 400 million.

Even with the expansion, Google isn't close to capturing the constantly expanding constellation of online content. By some estimates, there are 10 billion pages on the Web.

Nevertheless, the expansion and other recent improvement to Google underscore the company's determination to remain the Internet's most popular search engine -- a position coveted by formidable rivals such as Yahoo and Microsoft.

Sunnyvale, Calif.-based Yahoo has been drawing upon Google's index for the search results on its Web site since June 2000, but plans to end the partnership before April. Yahoo intends to rely exclusively on search technology it picked up last year after spending more than $2 billion to buy Inktomi and Overture Services.

Meanwhile, Redmond, Wash.-based Microsoft is spending millions to develop a sophisticated search engine to use on MSN.com in hopes of toppling Google as the king of search.

Google has been regularly upgrading its search engine since its late 1998 debut with a Web index of 25 million pages, but the potential threats from Yahoo and Microsoft have added urgency, said co-founder Sergey Brin, the company's president of technology.

"We have decided to put even more energy into our improvements and have turned up the notch on innovation a bit," Brin said in an interview Tuesday.

The upgrades are being made as Google mulls a widely anticipated initial public offering of its stock later this year. Brin declined Tuesday to discuss the possible IPO, expected to be one of the richest in nearly a decade.

A Google IPO could build a bigger war chest for its looming search showdown with Yahoo and Microsoft.

Google has built a sizable lead since Brin and fellow Stanford University graduate student Larry Page developed a new way to guide Web surfers to their desired Internet destinations.

Google's Web sites handled 35 percent of all Web searches in December, compared with 27 percent at Yahoo sites and 15 percent for Microsoft sites, according to the latest data compiled by comScore Media Metrix, a research firm.

AOL and other Web sites owned by Time Warner have a 16 percent share. Those sites largely rely on Google for their search results.

In its latest makeover, Google tweaked the closely guarded formula that determines which Web sites are most relevant to a search request.

Google has made five significant changes to its algorithmic formulas in the last two weeks, Brin said.

The revisions to the formula are even more important than the expanded Web index to e-commerce sites because a ranking on the first page of Google's search results typically generates much more customer traffic than on subsequent pages.

As Google covers more online turf, it also is digging deeper into Web pages. Roughly 40 percent of the Web pages scanned by Google weren't fully indexed until the latest improvements, Brin said. Now all but about 20 percent of the Web pages that Google covers are fully indexed.