Sources have told multiple news outlets that the European Union is readying antitrust charges against Google regarding the company’s AdWords ad placement service. The charges allege that Google blocked advertisers from moving their campaigns and data to competing ad services.

This would be the third antitrust charge levied by the EU against Google as part of a probe that was first announced five years ago. The first charge claimed that Google promoted its own shopping search results ahead of its competitors, with reports that the EU would bring down a 3 million Euro fine. The second charge alleged that Google required smartphone makers to pre-install Google and Chrome apps to access the company’s other apps.

The difference with this charge is that it directly affects Google’s most important revenue stream; the company’s advertising business accounts for roughly 90% of the $75 billion it generated last year.

Sources familiar with the investigation have said that the European Commission (the executive body of the EU) has asked for search advertising information from Google, and has also asked competitors to share information with Google as evidence. Both moves are typical before the Commission issues a formal charge.

Google’s AdWords places advertisements within (and often at the top of) Google search results as well as on Google-owned sites and other websites. Statistics say that for every $1 spent on AdWords, the average business generates $2 in return.