LAS VEGAS (AP) ¿ Casino operator Riviera Holdings Corp. said Friday it is voluntarily delisting its common stock, saying that it cannot meet required compliance standards by a required deadline. Riviera said it received a deficiency letter from NYSE Amex LLC, dated Monday. The letter said Riviera did not meet listing standards because of losses that are so "substantial" and an impaired financial condition, that it remains questionable whether the company can continue operating. Last month, Riviera said its losses narrowed in the first quarter because of a gain related to the fair value of derivatives. But Riviera's gaming operations were hurt by a tough economy that has consumers scaling back on spending and travel. Meanwhile, Riviera has defaulted on credit agreements and has previously said it might need to file for Chapter 11 bankruptcy protection. Riviera said it has been asked to submit a compliance plan by July 1 that advises how it will become compliant with listing standards by Nov. 27. But Riviera said it cannot take the necessary steps to satisfy the criteria within the time frame and will not submit a plan to gain compliance. The delisting is effective 10 calendar days after filing the required forms. Riviera expects the last day of trading will be around June 25. Shares dropped 39 cents, or 30 percent, to 90 cents.