NEW YORK (CNN/Money) - A quarter century of television advertising growth is expected to peak in 2006 when broadcast networks begin losing market share to the Internet and other emerging forms of advertising, according to a report released Monday.

As part of a broader study, Zenith Optimedia, a London-based unit of the French advertising giant Publicis Groupe (Research), predicted that television's share of the estimated $417 billion global ad market will begin to fall in 2007 as advertisers steer more of their marketing dollars to the Web.

Overall, the Zenith Optimedia report concluded that the robust global ad market of recent years will continue. The company raised its global ad growth projections each of the next three years, including a projected 2005 growth rate of 5.4 percent, to $371.4 billion. Previously, the agency had predicted 5 percent growth.

Still, Zenith Optimedia's 2005 global forecast was tempered given that the ad market will not benefit from the Euro soccer championship, the Olympic Games and the U.S. presidential elections, which contributed to a blistering 7.5 percent growth rate, to $346 billion, in 2004.

Signs of an overall healthy ad market are encouraging given some potentially worrisome indicators in the United States, including a recent spate of retail mergers, ongoing woes in the automotive sector, and the possibility raised last week by IBM's surprisingly negative earnings report that the technology sector is headed for trouble.

The broadcast networks could also be breathing a sigh of relief as they head into next month's "upfront," when they plan to sell roughly 80 percent of commercial time for the new television season that starts in September. Last year an estimated $9.5 billion worth of broadcast ads were sold during the upfront.

The rise of ad-zapping digital video recorders, alternative forms of entertainment and newer advertising media like the Internet are making network executives anxious.

Based on Zenith Optimedia's report, however, they don't have anything to worry about until 2007.

The estimated decline in television advertising as a percent of total market share -- from a peak 37.9 percent in 2006 to 37.8 percent in 2007 -- might seem trivial. But it will come after years of steady growth in television advertising since at least 1980.

Television's ascendancy has been driven by a slew of factors, including deregulation in Europe, the gradual decline of newspaper and magazine circulation in developed countries and better forms of audience measurement.

The only other year in which television's share of worldwide ad spending fell was in 2001. Zenith Optimedia suggested that the falloff then was an anomaly, the result of the dot.com bubble burst.

The dot.com implosion hit Internet advertising hard. But it has since rebounded as the economy has recovered and marketers have become savvier about the Web.

Internet advertising is still the second smallest form of advertising, sandwiched in terms of market share between outdoor and movie theater advertising, and is expected to hold that ranking through 2007.

As a percent of the total ad market, the Internet's share should increase from 3.6 percent in 2004 to 4.4 percent in 2007, Zenith Optimedia found.

The study tracked seven major advertising forums, including radio, magazines, and newspapers. Categories not covered included video game advertising.