E.U. antitrust regulators have asked Google's rivals if the internet search giant unfairly demotes local search competitors, according to a questionnaire seen by Reuters, a move which could lead to a fourth case against the Alphabet unit.

Google has been fined a total 6.76 billion euros ($7.7 billion) in the last 17 months for favoring its comparison shopping service and for using its dominant Android mobile operating system to reinforce its search engine market power.

The European Commission, which took the world's most popular internet search engine to task for these two anti-competitive practices, is wrapping up a third case which involves Google's AdSense advertising service.

The EU competition authority's interest in local search services followed a complaint by U.S. search and advertising company Yelp and rivals in the travel, restaurant and accommodation industries.

It sent questionnaires to Google rivals last month, asking for details of the company's practices and the impact on competing services between January 2012 to December 2017.

Regulators also wanted to know if rivals experienced an impact in the operation of their local services as a result of major search algorithm changes by Google, including the introduction of its Panda 4.0 algorithm.

Introduced in 2014, this algorithm determines what appears in Google search results.

Companies were also asked if Google's introduction of the Local Universal or One Box had a substantial impact on their local search services.

Local Universal is targeted at hotel ads while One Box, which is information and images outlined in a box, is a tool for local businesses to get more visibility in Google search results.

The Commission asked if Google used content from rival local search services such as reviews on Local Universal or One Box.

Google had no immediate comment.