Microsoft will be fined €497 million (£332m) by the European Commission on Wednesday for abusing its monopoly in computer operating systems, according to leaks.

The fine, which was set late last week when settlement talks with Microsoft broke down, was backed by national competition regulators from the 15 Union member states yesterday. Today, it will be discussed by senior aides to all 20 commissioners before being brought up at the EC's final meeting on the case tomorrow morning.

Microsoft will then be officially informed of the fine and sent a summary of the ruling by fax, shortly before competition commissioner Mario Monti holds a press conference to announce the decision.

The software giant, which was briefing journalists over the weekend that there would be no fine at all, has surprised no one by announcing the fine is too big. "In view of the absence of a clear legal standard under EU law, a fine of this size isn't warranted," said the company's spokesman in Brussels.

This angling was dismissed out of hand. ''We have already told Microsoft, many times, that a negative ruling will incur a fine,'' said Amelia Torres, Mario Monti's spokeswoman. ''A small company could claim it didn't know the rules but not one the size of Microsoft.''

Others however have pointed out that the proposed fine represents only one per cent of Microsoft's gross income, and the EC has the power to impose up to a 10 percent fine. A one percent fine is not enough to make the software giant change its ways, critics claim.

The Commission is widely expected to rule that Microsoft abused the monopoly position of its Windows operating system twice. First, by withholding vital information about Windows from server software manufacturers and second, by unfairly bundling its Media Player software into Windows.

As such, remedies are expected to include forcing Microsoft to open up its source code and make it release a version of Windows without the Media Player installed.

Some analysts said these remedies are more important than the fine, in terms of making an impact on Microsoft, as the company has over $50 billion (£27bn) in cash reserves and has already set some of that aside for covering legal costs.