LUXEMBOURG— Microsoft Corp. suffered a blow in its long-running battle with European Union regulators Wednesday, as Europe's second-highest court upheld a billion-dollar fine imposed in 2008 for failing to comply with an order to share product information with competitors.

The European General Court did reduce the fine slightly—to €860 million ($1.07 billion) from €899 million—but it rejected the software giant's argument that the European Commission, the EU's executive arm, wasn't justified in imposing the penalty for failing to obey orders four year earlier to open up the software market to rivals.

Microsoft said it was disappointed with the decision; the company's lawyers had argued during a court hearing in May 2011 that the fine, which at the time amounted to 2% of Microsoft's revenue, was undeserved and excessive.

The ruling comes after more than a decade of legal conflict between Microsoft and the commission, which began in 1998 when Sun Microsystems lodged an antitrust complaint over the Windows operating system.

In the course of its showdown with the EU, Microsoft has had to pay a total of €1.7 billion in penalties, paying €497 million in 2004 for anticompetitive practices, with a further fine of €281 million for additional breaches of competition rules.