The travel websites Expedia and TripAdvisor have filed complaints as part of a probe into whether Google is abusing its position on the Internet, a European Commission spokesman said on Tuesday.
With the complaints recently filed by Expedia and TripAdvisor "we have received 13 complaints as part of the antitrust probe we opened in 2010," Commission spokesman Antoine Colombani.
Sources familiar with the probe said the complaints concerned Google Flight, which like Expedia and TripAdvisor allows users to search for, and purchase, airline tickets online.
The Commission opened in November 2010 a probe into whether Google, the top web search engine, was rigging the online search market.
The EU competition watchdog said it had received several complaints by smaller companies of "unfavourable treatment" of their services in both unpaid and sponsored search results, the crucial listings that make the web navigable.
Competition authorities said they would also probing whether Google's own services -- including YouTube video, book-scanning or telephony -- were getting "preferential placement" when users punch in search queries, some of which may lead to consumer spending.
EU Competition Commissioner Joaquin Almunia said last week that the Commission's initial findings would be ready in April.
Explore further: Celebrities' nude shots removed from some websites