Microsoft has been formally accused of failing to comply with a binding agreement to give customers a choice among internet browsers.
In 2009, the European Commission said it suspected the software giant of using its dominant market position to foist its Internet Explorer browser on users.
The company agreed to create a screen where users could choose among competitors' browsers. The Commission accepted that concession and made the creation of a "browser choice screen" legally binding.
But in July, the Commission said the screen had not been displayed on many computers between February 2009 and July 2012, and millions of users may have been affected during that period. At the time, Microsoft said that a technical error was responsible.
EU Competition Commissioner Joaquin Almunia said any breach would be a "serious infringement," noting that the settlement between the EU and Microsoft had allowed the company to avoid lengthy anti-competition proceedings and being held liable.
"Companies should be deterred from any temptation to renege on their promises or even to neglect their duties," he said.
The company has apologised for the error and said it was working to make sure it did not happen again,
"We take this matter very seriously and moved quickly to address this problem as soon as we became aware of it," Microsoft said. "Although this was the result of a technical error, we take responsibility for what happened."
Microsoft will now be given four weeks to respond to the formal complaint and can seek an oral hearing. Once it has assessed Microsoft's defence, the Commission will rule.
The company could face a fine of up to 10% of its annual revenue if found in breach of antitrust law.