The European Commission is ready to impose a fine of 2m euros ($2.5m; £1.4m) a day on Microsoft.
Microsoft says it is working hard to meet Brussels's demands
The Commission is expected to rule that Microsoft has failed to fully implement its 2004 antitrust decision.
Under the ruling, Microsoft had to supply rivals with information about its Windows operating system.
On Monday, Microsoft said it had begun to provide the information Brussels had demanded, but the Commission has signalled the company acted too late.
In December, Brussels informed the software giant that it had failed to comply with the original ruling it issued in March 2004.
At the time of the warning, the Commission said Microsoft would face fines of up to 2m euros a day if it did not comply immediately.
As Microsoft has taken a further six months to begin handing over information, experts believe the Commission will take the view that the firm has been acting illegally since the warning.
Brussels is currently preparing a draft ruling that will be presented to national competition authorities on Monday, while an official ruling is expected on 12 July.
But in a statement the US giant said it had been "working hard" to meet the Commission's demands.
It added it expected to submit more information on 30 June and 18 July.
"Given that the technical engagement is producing results and that Microsoft has complied fully with every instruction given by the Commission and the Trustee, any fine would be unjustified and unnecessary," it said.
Under the landmark 2004 decision, Brussels found Microsoft was abusing its market leading position in the desktop software market to stifle competition.
The US group was then fined 497m euros ($613m; £344m) and ordered it to change how it sells its Media Player software. Brussels also ordered Microsoft to provide rivals with enough information to develop software that could run as smoothly as its own on servers running Microsoft's Windows operating system.
Since then, Microsoft has been involved in a legal tussle with the Commission about the information with its lawyers claiming that the Commission is forcing it to divulge valuable trade secrets.