The European Commission has warned Microsoft that it could face further fines for failing to comply with its anti-trust rulings.
Microsoft and Brussels are locked in a war of words
Brussels criticised the software giant for not changing its behaviour since it was fined 497m euros (£331m) in 2004.
In an escalation of its long-running battle with the US firm, Brussels said Microsoft had been seeking unreasonable prices from competitors for key data.
In response, Microsoft said the threat of further fines was unreasonable.
"In the 50 years of European antitrust policy, it's the first time we've been confronted with a company that has failed to comply with an antitrust decision," a Commission spokesman said.
The EU has sought to force Microsoft to disclose information that will allow other companies to make sure their software works properly with Microsoft's operating systems.
The latest move comes after the Commission not only fined the firm, but also ordered it to make a version of Windows available without Media Player software.
After its latest rebuke, Competition Commissioner Neelie Kroes said she was "not used to dealing with companies who drag their feet as Microsoft is doing".
"Microsoft is a company not used to losing."
But the firm said: "It is hard to see how the Commission can argue that even patented innovation must be made available for free."
Last year, Microsoft was told to pay daily fines adding up to 280.5 million euros over a six-month period, after it failed to adhere to the 2004 ruling.