The European Commission warned Microsoft on Thursday it faced further fines in its antitrust case for seeking unreasonable prices from software makers for interoperability information.

The latest formal charges could lead to new fines against the U.S. software giant, the Commission said in a statement.

The Commission found in 2004 that Microsoft violated European Union antitrust rules by failing to give information to other makers of small-business servers needed to compete with Microsoft's own products.

Microsoft has said it would be charging for interoperability information because it was based on its own innovative work.

"The Commission's current view is that there is no significant innovation in these protocols," European Competition Commissioner Neelie Kroes said in a statement.

"I am therefore again obliged to take formal measures to ensure that Microsoft complies with its obligations," she said.

Microsoft has four weeks to reply to the charges.

The Commission rejected 1,500 pages of information provided by Microsoft.

It said it is still reviewing whether information submitted earlier to help software makers develop programs that run smoothly with Microsoft's Windows operating system is complete and accurate.

Microsoft has appealed the original 2004 decision to a European court which is due to decide on the case this year.