WASHINGTON (Reuters) – The Justice Department should put conditions on Google Inc's proposed purchase of airline ticketing software company ITA Software Inc, Senator Herb Kohl, head of a senate antitrust panel, said in a letter on Wednesday.

Kohl said the $700 million deal could end up depriving travel websites such as Expedia, Orbitz, Kayak and TripAdvisor of critical software.

"Many of ITA's customers believe that access to ITA's technology is critical to competition in online air travel search because it cannot be matched by other players in the travel search industry," Kohl wrote in the letter to Christine Varney, head of the Justice Department's antitrust division.

The letter was dated December 1.

Google has said that it would honor contracts that ITA has with the travel sites, and Kohl asked that the Justice Department include that as part of a consent decree approving the transaction "to ensure that Google's promise is enforceable."

Kohl also referred to accusations that Google manipulated search results to steer consumers to its own services and to the fear that Google planned to start its own travel search service.

He urged the Justice Department, if it found the accusations "compelling," to consider conditioning any approval of the deal on a ban on biasing air travel search results.

Google argues that since it does not compete against ITA Software, the deal will not change existing market share in the online travel industry and will result in better search results for consumers.

"We've pledged to continue licensing ITA's search tools to other sites, and hope to drive more potential customers to airline and online travel websites," said Google spokesman Adam Kovacevich.

(Reporting by Diane Bartz; Editing by Richard Chang)