The Nuclear Regulatory Commission yesterday removed the major obstacle to startup of two controversial East Coast nuclear power plants by dropping a requirement that local and state authorities participate in emergency evacuation plans.

The NRC voted unanimously to consider licenses for reactors on a utility's emergency plan alone if the authorities refuse to cooperate, eliminating the main weapon of opponents to the Seabrook nuclear plant in New Hampshire and the Shoreham plant on Long Island. Both facilities, long ready for operation, have been blocked by local and state officials' refusal to cooperate in emergency plans.

Commissioners said that the ruling does not assure licenses for Seabrook, Shoreham or any other plant. Utilities still must demonstrate their ability to protect the public health and safety in the absence of local and state participation in planning.

But, they stressed, in reviewing such blueprints, including the pending Shoreham and Seabrook plans, they will assume cooperation by local and state authorities in a real crisis, even if they refuse to participate in the planning.

"It's no panacea for any particular plant," NRC Chairman Landon W. Zech Jr. said of the new rule. "It just provides a vehicle {for licensing plants} when state and local governments won't cooperate in planning. We do assume state and local governments will do whatever is necessary to protect their citizens."

Spokesmen for the utility managers of the Seabrook and Shoreham plants, both $5 billion facilities, welcomed the ruling as a way of breaking the regulatory logjam that has resulted in costly delays to full operations.

But opponents in environmental groups and state and local governments strongly criticized the decision for flouting one of the most important regulatory reforms to follow the 1979 accident at Three Mile Island. Adopted in 1980, it required utilities to plan for evacuation of all communities within a 10-mile radius of their plants, and to conduct drills to show that the plan would work.

Massachusetts Gov. Michael S. Dukakis (D), a candidate for president whose state lies within the evacuation range of Seabrook and has refused to participate in emergency planning, threatened to challenge the ruling in court.

"The NRC has in effect become the fox in the henhouse, lobbying for the industry it is mandated to regulate," he said.

Similar opposition was voiced in New York, where the state and Suffolk County governments have refused to cooperate in evacuation plans for Shoreham, which lies along Long Island Sound.

A spokesman for New York Gov. Mario M. Cuomo (D) said a new standard "that changes the rules of the game is something we find repugnant."

"The NRC has failed," said Suffolk County Executive Michael LoGrande. "We will not tolerate indifference to public safety."

New Hampshire Gov. John H. Sununu (R) has submitted evacuation plans for Seabrook, 40 miles north of Boston. But Dukakis has declined to cooperate, contending that no plan could adequately assure public health and safety in event of a reactor meltdown.

Cuomo and Long Island authorities argue that quick evacuation is not practical from Shoreham, which lies 75 miles northeast of New York City and has the normally congested Long Island Expressway as its main avenue of escape.

Both Public Service Co. of New Hampshire, primary owner Seabrook, and Long Island Lighting Co., Shoreham's owner, have filed emergency evacuation plans with the NRC without the participation of the New York and Massachusetts governments. Shoreham, essentially ready for operation in 1984, has been running at low power for two years. Seabrook was allowed to load nuclear fuel one year ago.

The regulation approved by the five-member NRC said that state and local participation is "highly desirable, and indeed is essential for maximum effectiveness of emergency planning and preparedness."

But Congress, which required cooperative planning in 1980, "did not intend that the absence of such participation should preclude licensing of substantially completed nuclear power plants where there is a utility-prepared emergency plan that provides reasonable assurances of adequate protection to the public," the NRC said.

In an accompanying "discussion," commissioners said the rule establishes a "framework" in which a utility seeking an operating license can show that its emergency planning is "adequate" even if local and state authorities refuse to cooperate.

In assessing a utility's plan, the NRC said, it is "appropriate" to "take account of that self-evident fact" that state and local authorities would cooperate with the utility in an actual emergency.

Staff writers Howard Kurtz in New York and Michael Rezendes in Boston contributed to this report.