The first test of the Microsoft antitrust ruling could be looming following complaints from the US Justice Department and 19 states over the company alleged use of its OS dominance to influence where users buy music online.

The dispute involves a built-in Shop-for-Music-Online link in Windows that uses Internet Explorer. The litigants believe the feature may be inconsistent with the antitrust settlement. Microsoft does not agree. If the issue is not resolved by the end of the week "it could become the first test of Microsoft's landmark antitrust settlement that was approved by a federal court in October 2002", Associated Press reports.

The dispute has already been before the three-person panel of technical experts convened under October's settlement. The panel must oversee that terms of the settlement are met. Under the settlement, Microsoft agreed to improve the opportunity for rival software suppliers to compete against its own software offerings.

The battle over online music sales was disclosed to the public yesterday, while papers were filed with the court last week.