The Nuclear Regulatory Commission yesterday said it would issue an order next month shutting down the Indian Point atomic power reactors, 35 miles north of New York City, unless their operators produce a satisfactory plan for protecting nearby residents in an emergency.

The unanimous decision marked the first time the commission has threatened to suspend the operation of atomic power plants for lack of an adequate plan to evacuate or shelter residents who live within 10 miles of the facility in the event of a severe nuclear accident.

The emergency-planning rule was adopted in 1979, after the nuclear power accident at Three Mile Island in Pennsylvania.

The NRC yesterday also warned the owners of the Maine Yankee atomic power plant in Wiscasset that their emergency plan had been found defective and will have to be corrected within 120 days.

Two similar warnings were issued earlier to Indian Point. About 290,000 people live in the 10-mile zone around the two large reactors, Indian Point 2 and 3, in Buchanan, N.Y., on the east bank of the Hudson River.

This zone, largely consisting of Westchester County, site of the plant, and Rockland County, on the west bank of the river, is the most densely populated area around any nuclear power plant in this country.

The Federal Emergency Management Agency last month advised the NRC for the third time that emergency plans for Indian Point "continue to be significantly deficient," and "FEMA cannot assure that the public health and safety can be protected" in the area around the plant.

A major deficiency cited by FEMA is the refusal of Rockland County to participate in evacuation exercises. The county has rejected the emergency plan prepared by Indian Point's owners as unworkable and is trying to formulate its own plan, a process expected to take another year.

A second major factor cited by FEMA is uncertainty over whether bus drivers in Westchester County would be willing to remain on the job and help evacuate residents in the event of an accident.

The NRC, in issuing its order yesterday, recognized that "to some extent these problems are beyond the power of the licensees who own the plants to solve."

But the commission noted that twice--in 1981 and last year--it had warned the owners of the plants that, "based on FEMA's findings of significant deficiencies in planning standards, the NRC emergency planning regulations were not being met."

"At this point, the NRC must consider the shutdown of the plants pending correction or mitigation of the two major deficiencies," the commission said in its order.

The NRC gave the owners two weeks to explain in writing why the reactors should not be shut down, and set oral arguments for May 26. Unless the commission is satisfied that the problems have been solved, the NRC said it would issue an order by June 9 "promptly suspending operation" of the plants.

Indian Point 2, an 873-megawatt reactor that went into service in 1974 and is owned by Consolidated Edison, currently is operating. Indian Point 3, a 965-megawatt reactor that started up in 1976 and is owned by the New York Power Authority, is shut down for refueling but is expected to be back in operation shortly.

Indian Point 1, a smaller reactor that is technologically obsolete, has been shut down for years.

Although NRC Chairman Nunzio Palladino expressed optimism yesterday that "a workable solution is possible," officials of municipal- ities near the plant were skepti- cal.

Joan Holt, a spokesman for the New York Public Interest Research Group, which has asked the NRC to close the plants, said she saw little prospect that officials "are going to be able to solve in three weeks the problems they haven't been able to solve in three years."

But Carl Patrick, a spokesman for the New York Power Authority, said: "We will be willing to work with the commission and do whatever is necessary." Pat Richardi, a Consolidated Edison spokesman, said: "We are sure the two areas will and can be satisfactorily resolved."