The EU's executive body, the European Commission, has opened an investigation into whether Microsoft has kept the antitrust commitments it made in 2009, and warned that penalties for non-compliance would be "severe."

EU Competition Commissioner Joaquin Almunia told reporters Tuesday that indications are that Microsoft, the company that makes the Windows operating system, has failed to provide customers with a screen from which they could choose different internet browsers other than its own Internet Explorer.

Almunia said it appeared that the choice screen, promised by Microsoft in 2009 following an antitrust case, has not been provided since February 2011, meaning 28 million customers may not have seen it.

In a statement, Microsoft conceded it had "fallen short" of its responsibility, while acknowledging that it was required to provide the "browser choice screen," or BCS.

"Due to a technical error, we missed delivering the BCS software to PCs that came with the service pack 1 update to Windows 7," the statement said.

The company said that PCs running the original version of Windows 7, as well as Windows XP and Windows Vista, did have the screen.

"While we have taken immediate steps to remedy this problem, we deeply regret that this error occurred and we apologize for it," Microsoft said.

Microsoft submitted a report to the Commission in December saying the browser choice screen was being provided as required. In its statement Tuesday, the company said it believed at the time that was the case.

Almunia said this would be the first time that this type of legally binding agreement has not been complied with.

"Needless to say, we take compliance with our decision very seriously," he said. "If the infringement is confirmed, there will be sanctions."

All told, the European Union has so far fined Microsoft 1.64 billion euros ($2 billion).