Expedia Inc. and Orbitz Worldwide Inc., which squeezed agents out of the travel business, agreed to merge Thursday as they struggle to hang on to the middle of a market that is now squeezing them.

On Thursday the online rivals agreed to a deal that would bring many of the top U.S. travel-booking sites under one roof. The $1.34 billion transaction is sure to spark grumbling from hotel and airline operators, who are wary of having even bigger companies standing between them and their customers.

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