European regulators have slapped Microsoft (NASDAQ:MSFT) with a hefty fine for breaking an agreement over the way Windows users may choose their Internet browser.

On Wednesday, the European Commission imposed a $731 million penalty on the software giant, saying it had breached a 2009 agreement under which it agreed to offer users a choice of Internet browsers instead of promoting its Internet Explorer as the default browser in its software. While the fine was unprecedented, the Commission could have demanded as much as 10% of Microsoft’s annual revenue, Reuters noted.

In a statement, Microsoft apologized and said it was taking steps to avoid the “mistake” in the future. The company did not signal whether it would appeal the fine, but it is seen as unlikely to do so.

The Commission said that between May 2011 and July 2012, Microsoft released software that used Internet Explorer as the default browser without offering users a chance to choose rival browsers. 15 million users were affected by Microsoft’s actions.

Microsoft has been involved in a number of disputes with the Commission over the past ten years and has been fined a total of 2.2 billion euros, including the latest penalty.

Other large American technology and Internet firms, including Google (NASDAQ:GOOG) are currently under investigation by the European regulators.

Shares of Microsoft fell more than 1% in Wednesday morning trading.