Google, a longtime ally of travel sites, is now a powerful rival

Google has long been one of the biggest drivers of new business for travel websites like Expedia and Tripadvisor. Some of them say the Alphabet Inc. unit is also a big competitive threat.

Berlin-based HomeToGo GmbH is an example of the bind facing many travel businesses. The vacation-rental website depends in part on Google to direct people to its service, but it says business has been affected by the search company’s placing a box of listings from third-party travel sites atop many pages of search results for vacation rentals.

In Google searches conducted this past summer, the rate at which people clicked when HomeToGo was the top regular result fell as much as three-quarters when Google’s vacation-rental listings box was present above, compared with when it wasn’t, according to calculations HomeToGo provided to The Wall Street Journal.