Hewlett-Packard (HP) is suing Oracle over a chip dispute in the latest clash between the two former allies.
The lawsuit accuses Oracle of behaving illegally in deciding that future versions of its database software will not support one line of HP servers.
The servers use an Itanium chip that Oracle insists is being phased out.
HP claim that Oracle's decision to stop the support will hurt its clients - Oracle has denied the claims, calling the lawsuit "malicious and meritless".
Relations between the two companies have deteriorated in the past year.
HP's former boss, Mark Hurd, joined Oracle almost a year ago.
HP had then threatened court action, claiming that Oracle would "exploit the knowledge of HP's strengths and weaknesses" as a result of hiring Mr Hurd.
The current dispute is the result of a combination of factors.
It centres around the future of the Itanium chip manufactured by Intel and the growing rivalry between the two companies in the field of server hardware market.
In March, Oracle announced that upcoming versions of its database software would no longer support HP servers that use the chip.
It said it had evidence that Intel was planning to phase out the product, hence it did not plan to provide software that supports the processor - something Intel has denied.
But Oracle said in a statement that: "Intel's plans to end-of-life Itanium will be revealed in court".
HP says Oracle's real concern is to try to force customers to buy the Sun Microsystems servers manufactured by it.
The company claimed Oracle was using "strong-arm tactics to coerce customers into replacing their HP servers with Sun servers they do not want".
HP said it had sent a legal notice to Oracle last week, but a lack of response from Oracle had left it with no choice but to take legal action against its former ally.
"The silence from Oracle is deafening. We are very disappointed it has come to this," said Bill Wohl of HP.