BRUSSELS — The European Union’s antitrust watchdog is expected to find Alphabet Inc.’s GOOGL, +0.95% GOOG, +0.92% Google illegally abused the dominance of its Android operating system for mobile phones, issuing a multibillion-euro fine and ordering changes to the company’s Android-related business practices, people familiar with the matter said.

The European Commission, the bloc’s executive arm, is expected to find that the California-based company’s actions allegedly thwarted potential competitors to safeguard its mobile-advertising business.