In a major challenge to the travel industry’s leading websites, search engine giant Google Inc. launched today a new service allowing users to shop for plane tickets as easily as typing a Google search.

Google’s flight search offering is based on technology from ITA Software Inc. of Cambridge, which Google acquired in April for $700 million. Fearing Google would dominate the online travel industry, a number of travel websites formed a coalition in October to oppose Google’s acquisition of ITA. Federal antitrust officials approved the deal only after Google agreed to a number of restrictions aimed at preventing the giant search company from seizing total dominance of the online travel market.

Among the websites that opposed the ITA deal was Kayak.com, a Norwalk, Conn. company with a major software development facility in Concord. Yesterday, Kayak chief marketing officer Robert Birge said his company is well-positioned to fend off its newest competitor. “We know Google is a very prominent brand, but we also know they haven’t been successful in everything they’ve done,” Birge said. ”Right now, we’re very confident in our ability to compete.”

But Carroll Rheem, director of research at PhoCusWright Inc., a travel industry consulting firm in Sherman, Conn., said Google’s flight search service is a significant threat to Kayak and other rival flight booking services, like Expedia.com and Travelocity.com.“It’s the day all the online travel agencies have been dreading,” Rheem said.

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