MOUNTAIN VIEW, Calif. ( TheStreet) -- European antitrust authorities have opened a preliminary probe into complaints by three European Internet companies about Google's ( GOOG) tactics of lowering their rank in search results that appear on Google sites, Google revealed.

Julia Holtz, Google's senior competition counsel, wrote in a blog post that the EU has received complaints from a U.K. price comparison site, Foundem, a French legal search engine called ejustice.fr and Microsoft's ( MSFT) Ciao! from Bing.

Foundem argues that Google's algorithms demote their site in the search results because it is a vertical search engine and so a direct competitor to Google. Ciao! has complained about Google's standard terms and conditions.

"While we will be providing feedback and additional information on these complaints, we are confident that our business operates in the interests of users and partners," Holtz wrote.

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