Silicon Valley is bracing itself for fireworks as a long-running intellectual property and industrial espionage dispute between two of the most powerful names in technology finally reaches court.

Oracle is suing its arch-rival SAP for exploiting what it says were illegal downloads of Oracle software code three years ago, and the case has now dragged in another technology titan, Hewlett-Packard, which hired the former SAP boss Leo Apotheker to be its chief executive a month ago.

And on the eve of the trial, HP accused Oracle and its chief executive Larry Ellison, of "harassment" for implicating Apotheker in the alleged thefts and threatening to call him to the stand.

Oracle sued SAP in March 2007, alleging "corporate theft on a grand scale".

Employees at a now-defunct SAP subsidiary gained "repeated and unauthorised access" to a password-protected website for Oracle customers and downloaded thousands of Oracle software products and other confidential materials.

SAP admitted "inappropriate downloads" of Oracle software fixes and other support documents, but denies that it profited from stolen intellectual property. It denied more of Oracle's claims than it admitted, setting the stage for the legal showdown that opened with jury selection yesterday.

Ellison, in a piece in the Silicon Valley/San Jose Business Journal, dismissed HP and SAP's claims that Apotheker was not involved, and would present evidence at trial that he was. Unless, he said, HP "decide to keep their new CEO far, far away from HP headquarters until that trial is over".

HP responded: "Given Leo's limited knowledge of and role in the matter, Oracle's last-minute effort to require him to appear live at trial is no more than an effort to harass him and interfere with his duties and responsibilities as HP's CEO."

HP hired Apotheker to replace Mark Hurd, who was fired for breaking the company's code of conduct over expenses claims for dinners with a contractor, who later accused him of sexual harassment (claims he settled privately with her). Ellison then hired Hurd to be his deputy at Oracle.