U.S. regulators are preparing for a possible antitrust challenge to Google Inc's planned $700 million acquisition of airline ticketing software company ITA Software, according to a media report.

Officials at the U.S. Department of Justice have not decided whether to sue to block the deal, Bloomberg reported on Thursday, citing anonymous sources.

The report said Google triggered the government's preparations for a lawsuit in December when it invoked a provision of a federal law that requires the government to decide if it would challenge a deal within 30 days, the sources told Bloomberg.

A Google representative did not return requests for a comment.

(Reporting by Alexei Oreskovic. Editing by Robert MacMillan)