The European Union is threatening to impose a stiff fine on U.S. technology giant Microsoft because the company failed to give European customers a choice of web browsers.

Microsoft apologized on Tuesday for what it called a “technical error” in failing to offer 28 million European users of its Windows 7 operating system a choice of Internet browsers other than the company's Internet Explorer. The U.S. software company — when it settled an antitrust case against it three years ago — had agreed to offer multiple browser options, but has not done so since February 2011.

The company said it has taken steps to fix the problem and deeply regrets that it occurred. Microsoft offered to add 15 months to the 2014 date it had originally agreed to list the choice of browsers.

EU Competition Commissioner Joaquin Almunia said that if investigators determine that Microsoft failed to comply with its earlier agreement, the company could face “severe consequences.” The company was already fined more than $1 billion in the antitrust case.