NEW YORK - Business software maker PeopleSoft Inc. (PSFT.O) on Thursday said its board unanimously rejected an unsolicited takeover offer from Oracle Corp. (ORCL.O), calling the bid inadequate and also citing antitrust concerns.

PeopleSoft said in a statement that the multibillion-dollar offer would face lengthy antitrust scrutiny, with a significant likelihood that it would not receive the required regulatory approval.

The Pleasanton, California, company also said the $16-a-share bid, announced last Friday, undervalues PeopleSoft, whose stock is trading above $16.

Oracle bid for PeopleSoft only a few days after PeopleSoft made a bid for another rival, J.D. Edwards & Co. (JDEC.O). A union with J.D. Edwards would have pushed PeopleSoft above Oracle in terms of market share for business software.

PeopleSoft said the unsolicited and hostile nature of the Oracle offer ``is designed to disrupt the company's strong momentum at significant cost to PeopleSoft's customers.''

PeopleSoft shares opened slightly higher, up 3 cents to $17.65, on the Nasdaq stock market, while Oracle was up 12 cents at $13.38.