The Nuclear Regulatory Commission declined yesterday to issue a low-power license for New Hampshire's Seabrook nuclear power plant and refused to approve a boost to 25 percent power for the Shoreham nuclear plant on Long Island.

The action was a setback for the plants and at least a temporary victory for state and local officials, who contend that evacuation of the densely populated areas around the plants would be impossible after an accident.

The five-member commission voted, 3 to 2, to deny a low-power license to Seabrook, an 1,150-megawatt reactor that is owned by a dozen New England utilities and was completed last year at a cost of $5 billion.

The panel voted unanimously to deny Shoreham permission to increase power generation, despite warnings from its owner, Long Island Lighting Co. (LILCO), that New Yorkers will suffer power shortages this summer unless the 809-megawatt reactor is allowed to produce more electricity.

NRC spokesman Frank Ingram said the commission decided that LILCO's request "introduced new material and factual issues" that neither the utility nor the NRC staff could resolve. The "factual issues" were not identified.

Regarding Seabrook, he said, the commission determined that an emergency evacuation plan submitted by the utility was inadequate. The plan was to cover Massachusetts communities within the reactor's 10-mile emergency planning zone.

New Hampshire Gov. John H. Sununu (R) has submitted evacuation plans for his state, but Massachusetts Gov. Michael S. Dukakis (D) has declined to cooperate. Seabrook's primary owner, Public Service Co. of New Hampshire, submitted the Massachusetts plan on its own last month.

"The plan itself was simply deficient. It was not a bona-fide plan," NRC Commissioner Lando W. Zech Jr. said. " . . . It simply does not meet our standards."

Commissioners James K. Asselstine and Frederick M. Bernthal joined Zech in voting against the low-power license, which would have let Seabrook run at 5 percent capacity. Commissioners Kenneth M. Carr and Thomas M. Roberts voted in favor.

Seabrook opponents have fought the low-power license vigorously, arguing that producing electricity there would contaminate the plant with radioactivity and preclude its conversion to coal or other fuels.

Shoreham is operating at low power, but opponents have resisted increasing its output on grounds that no emergency plan could effectively evacuate Long Island after an accident.

Under NRC rules, full-power licenses cannot be granted to reactors without emergency plans. The commission is considering a rule change that would permit licenses for plants with plans it finds acceptable, even if local officials decline to participate, but the proposal has generated controversy in Congress.

New York Gov. Mario M. Cuomo (D), who also has refused to cooperate in emergency planning, said yesterday that increasing the reactor's output "would have definitely presented a threat to the health and welfare of the people of Long Island, especially since there is no approved evacuation plan . . . . "

Zech said the Shoreham vote was "not . . . on the merits" of the plant or its evacuation plans. He said the NRC does not have time to conduct hearings on new issues raised by LILCO's request before summer's end, suggesting that it does not intend to reconsider the matter for several months.