European Union regulators are in preparations to challenge Google's Android mobile operating system after an investigation of the company's Internet search practices, which lasted for almost four years.

Reuters cited two sources that said the case being prepared will focus on whether Google's Android is abusing its 80 percent market share for the promotion of its own services over those from the company's competitors.

Any company that manufactures mobile devices is allowed to use Android for their products. However, mobile phone makers that seek to use the latest version of the mobile operating system are required to pre-install a minimum number of services by Google on the devices.

The European Commission has accelerated its investigation on the case recently, with questionnaires being sent to participating companies that are looking for more details compared to the past questionnaires sent in 2011 and 2013.

The said questionnaires asked the respondent companies whether Google requires that there will be no pre-installed apps or services on mobile devices that are in direct competition with Google apps or services such as the company's search engine, maps and app store.

Respondent companies are also requested to submit all e-mail messages, faxes, letters, phone call and meeting transcripts, and presentations related to such agreements with Google to as far back as the year 2007, suggesting that the Commission is seeking to learn if Google's actions have been long-term. 

Respondent companies are given until early September to accomplish the questionnaire and submit all requirements.

A spokesman for outgoing competition commissioner Joaquin Almunia said that the commission is searching for evidence relating to "undue restrictions to competition."

"Our preliminary investigation concerning the Android ecosystem is ongoing. We continue to gather information. We are looking into whether there are undue restrictions to competition in this area," the spokesman said. 

The upcoming challenge on Android will add to the regulatory issues that the company is facing in Europe, which accounts for over $30 billion in digital advertising expenditures in 2013.

The Commission will likely begin its formal probe on Android once the investigation on whether Google ranks its own services at a higher position compared to services of rivals in search results is completed.

Google is also involved in the implementation of the "right to be forgotten" ruling enforced by the European Commission, which allows people to request Google and other search engines to put down links for content that the people don't want to be found when searching online.