The European Commission has begun an antitrust investigation into search giant Google.
"The Commission can confirm that it has received three complaints against Google which it is examining," the European Union's top antitrust authority said.
It added that this doesn't amount to a formal investigation "for the time being".
The Commission informed Google earlier this month, it said, and asked the company to comment on the allegations. It will co-operate closely with the national competition authorities from the 27 EU member states, it said.
The complaints filed with the Commission came from UK price comparison site Foundem, a French legal search engine called ejustice.fr and a German search site called Ciao that was recently acquired by Microsoft, Google said in a blog. The Commission did not name the companies.
Google pointed out in its blog that Foundem is a member of a trade group called iComp, which is largely funded by Microsoft.
"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, as well as in line with European competition law," Google said.
The search giant implied, but did not say outright, that Microsoft had a hand in two of the complaints.
"Regarding Ciao, it was a long-time AdSense partner of Google's, with whom we always had a good relationship," Google said in its blog post. "However, after Microsoft acquired Ciao in 2008 (renaming it Ciao from Bing) we started receiving complaints about our standard terms and conditions. They initially took their case to the German competition authority, but it now has been transferred to Brussels."
Microsoft said it was not surprised to see competition officials looking at the online advertising market, given its importance to the internet and "the dominance of one player". It said it had not been notified yet by the Commission about the Google inquiry.
"In the meantime, we continue to co-operate with the German government's investigation into complaints brought by Ciao, the German Newspaper Publishers Association, Association of German Magazine Publishers, and Euro-Cities, a Berlin-based online mapping company," Microsoft said in a statement sent via email.
James Niccolai contributed to this story.