The European Commission has decided to increase the daily penalty against Microsoft for failing to honor the antitrust ruling against it, to €3 million a day from €2 million a day, a person close to the Commission said on Tuesday.

The Commission is expected to fine the software company up to €2 million (£1.38 million) a day on Wednesday, covering the period from December 15 last year to the present. That fine could total €400 million.

The increased daily fine will be applied moving forward if Microsoft continues not to comply with the Commission's ruling, the person close to the Commission said, speaking on condition of anonymity.

As part of the 2004 antitrust ruling against Microsoft, the company was ordered to reveal technical information about its Windows operating system to rival software makers. Withholding the information amounted to an abuse of the dominance of Windows, the Commission ruled.

Microsoft claims it thinks the Commission’s demands are not clear. Last quarter it said it finally understood what the Commission was looking for, following an oral hearing with the Commission, competitors and Neil Barrett, the monitoring trustee overseeing Microsoft’s compliance.

Last week it said it is working hard to meet a July 18 deadline for the final submission of the technical information. It claims the deadline was set by the Commission, the monitoring trustee, and Microsoft itself.

However, the Commission denies it had anything to do with setting the deadline. “There was no joint setting of deadlines. There was no agreement on the timetable,” the person close to the Commission said.

The Commission will take at least two months to assess the final instalment of technical information to be submitted on July 18. If it is deemed satisfactory, the few days between Wednesday and the final submission of the technical information will be waived and the Commission will not impose the higher fine.

“The Commission doesn’t want to fine Microsoft again. It wants the fine for the period from December 15 to be a sufficient deterrent,” the person said.

But he stressed the possible need for a greater deterrent. “The Commission has no guarantee that the information submitted on July 18 will be adequate. It will have to look at it and if it isn’t enough, then the increased daily fine will be applied,” he said.

Neither the Commission nor Microsoft were immediately available to comment.