NEW YORK -- New York environmental regulators have denied a key water-quality certification Entergy Corp. needs to extend by 20 years its license to operate the 2,000-megawatt Indian Point nuclear-power plant.

The New York Department of Environmental Conservation said in a letter to Entergy dated April 2 that the two units of the plant "do not and will not comply with existing New York State water quality standards," even with the addition of a new screening technology favored by Entergy to protect aquatic life. The plant's...