BRUSSELS—The European Union's antitrust regulator said Tuesday that Microsoft Corp. had failed to keep commitments it made in response to a probe into the way the software maker tied Windows users to its own Web browser, putting it on the line for more fines.

Microsoft in late 2009 promised to offer users a choice of alternatives to its own Internet Explorer Web browser that comes preinstalled on most Windows personal computers. In practice, that means users in the EU should see a window pop up on their computer screen, allowing them to choose from as many as a dozen different browsers. The European Commission, the EU's competition watchdog, says that "ballot screen" helped break Internet Explorer's dominance in the EU, where it now ties for users with Google Inc.'s Chrome and Mozilla Firefox.

The 2009 browser settlement came after more than a decade of back and forth between Microsoft and the commission over various restrictions the company imposed on other software makers wanting to work with its Windows operating system—and after it was slapped with fines totaling some €1.64 billion, or $2.01 billion.

But on Tuesday, EU Competition Commissioner Joaquín Almunia said his team is now investigating why Microsoft didn't include the "ballot screen" in versions of its Windows 7 software sold in the EU since February 2011. Because of that, some 28 million Windows users may not have seen the ballot screen at all, he added.

Microsoft acknowledged it "missed" displaying the screen in some Windows versions, blaming a technical error. "We have fallen short in our responsibility to do this," it said in a written statement. "While we have taken immediate steps to remedy this problem, we deeply regret that this error occurred and we apologize for it."