Action against McDonald's was urged in February by umbrella organisations for unions representing millions of workers in the United States and Europe, and charity War on Want, who accused it of avoiding around 1 billion euros ($1 billion) in tax between 2009 and 2013 by routing revenue through a Luxembourg unit.
European Competition Commissioner Margrethe Vestager could announce the investigation as early as Thursday, the sources said, marking her latest action against multinationals.
In October, Vestager ordered the Dutch government to claw back up to 30 million euros in back taxes from U.S. coffee chain Starbucks and told Luxembourg to do the same for Fiat Chrysler Automobiles. Cases against Apple's Irish tax deal and Amazon's arrangement in Luxembourg are pending.
Commission spokesman Ricardo Cardoso and Luxembourg's finance ministry both declined to comment. No one at McDonald's could immediately be reached for comment.
The EU competition enforcer has been examining McDonald's case for some time now, said a third source familiar with the issue.