The evidence from an IBM executive this week represents an opportunity for US government lawyers to re-focus the Microsoft antitrust trial on a central accusation.

Like the computer companies testifying before it, IBM has repeated the damaging claim that Microsoft used its power over its Windows operating software to suppress competition.

The IBM testimony - from Mr Garry Norris, sales and marketing director in networking hardware - underlines the argument that Microsoft has engaged in extending its monopoly power, just as it is alleged to have done with Netscape Communications, Intel and Apple Computer.

Mr Norris's evidence comes in the last stages of a trial in which both sides are rebutting each other's arguments. At this point, any new allegations are increasingly hard to dismiss and will be uppermost in Judge Thomas Jackson's mind as he crafts his initial findings.

But the irony in IBM's evidence is obvious to many on the government's side.

Several government attorneys and expert witnesses - including its leading lawyer David Boies - were involved in IBM's own protracted antitrust battle with the US government.

The IBM case was finally dropped, after more than a decade in court, under the Reagan administration in 1982.

Furthermore, the rift between IBM and Microsoft is well established and clouds any judgment of what Microsoft calls complex negotiations.

Microsoft led the development of the personal computer industry that reversed IBM's fortunes in the 1980s.

Nevertheless, Mr Norris's testimony has provided compelling evidence, not least because of his meticulous notes and records of various meetings and conversations with Microsoft around the launch of Windows 95.

Mr Norris, responsible for negotiating with Microsoft over Windows 95, revealed that IBM was consistently pressured to cease competing with the world's largest software company.

The pressure came from Microsoft's control of the prices it charged for Windows 95.