HP has filed a lawsuit against its former chief executive, Mark Hurd, which seeks monetary damages and assurances that HP's secrets are safe.
The move comes after Oracle appointed Hurd to its board and gave him the role of co-president in charge of sales, marketing and support, according to US reports.
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Hurd signed agreements to protect HP's trade secrets and confidential information, and HP intends to enforce those agreements, the company said in a statement.
The company claims Hurd's position at Oracle will lead inevitably to him divulging confidential information.
HP is seeking written guarantees to ensure Hurd's compliance with the agreements he signed. It is also seeking monetary damages for "willful and malicious conduct".
A similar lawsuit was filed by IBM in October 2008 against former executive Mark Papermaster, when he joined Apple.
That suit was settled in January, 2009, but required Papermaster delay joining Apple until April and to comply with his IBM employment agreement for several months.
Hurd resigned after an internal investigation at HP alleged he had made inaccurate expense claims and concealed a relationship with events contractor Jodie Fisher.
Hurd has denied making any advances towards Fisher. He has also insisted that he did not prepare his own expense claims and did not try to conceal his outings with Fisher.
Oracle chief Larry Ellison came out in strong support of Hurd, criticising the HP board's actions because it had found no truth in Fisher's sexual harassment claims.
"The HP board just made the worst personnel decision since the idiots on the Apple board fired Steve Jobs many years ago," he said in a letter to the New York Times.