The Supreme Court, in a decision which could help traditional unions fight off automation, yesterday ordered the National Labor Relations Board to reconsider the legality of work rules negotiated to keep labor peace on East Coast docks in the face of containerization.

The NLRB had invalidated the rules in 1978, on the grounds that they illegally took jobs from nonunion freight loaders. That invalidation upset a delicate harmony between the longshoremens union and the shippers, raising the prospect of another strike this summer.

The justices did not approve the work rules yesterday, however. They simply ruled that the NLRB used improper standards in evaluating them. The new standards set up by the court yesterday could help other unions faced with similar disputes, some lawyers believe.

The rules, negotiated by unions in New York, Baltimore and Hampton Roads, were designed to stem a precipitous loss of work for longshoremen since the advent of containerization. Containerization permits one swoop of one crane to do the work of dozens of men. Numerous articles for shipment are loaded into massive, trailer-like boxes, called containers, and hoisted to and from the ship. Before containerization, each article had to be loaded separately.

The work rules were aimed at freight consolidators, a newer breed of business, often nonunion, that competed with the longshoremen by stuffing containers away from the docks and stripping them on delivery.

The rules, in effect, gave the unions jurisdiction over this stripping and stuffing within a 50-mile radius of the piers. Shipping lines could be fined for violating the rules. The unions also could go through the purposeless task of unloading the containers and re-stuffing them.

The NLRB rejected the rules, saying that rather than preserving traditional longshoremens work -- which might have been legal -- the rules expropriated work that belonged to others, the consolidators and truckers.

Justice Thurgood Marshall, writing for the majority, ruled that the NLRB was too narrow in making its determination. He said the board examined only the traditional work of consolidators and longshoremen to reach its decision, but should examine "the traditional work patterns that the parties are allegedly seeking to preserve, and of how the agreement (in this case, the rules) seeks to accomplish that result under the changed circumstances created by the technological advance.

"The board's determination that the work of longshoremen has historically been the loading and unloading of ships should be only the beginning of the analysis," Marshall wrote.

He said the next step is to look at the collective bargaining agreement and determine its objective --whether it is designed to preserve longshoremen's traditional work and how far it goes in preserving it. Marshall noted that the ILA agreement was a compromise which permitted "the great majority of containers to pass over the pier intact."

Some lawyers speculated that Marshall's opinion would have a major impact on future automation disputes in favor of unions. Marshall went beyond an examination of the work involved, they said, and into a study of whether an effective compromise was reached by the union in adjusting to automation. That changes the law's purpose from "work preservation" to employe preservation," it was suggested.

The justices took pains, however, to point out that they are not deciding the merits of the dispute. The board must reconsider the issue "in relation to a proper understanding of the work at issue," they said.

The court thus avoided what could have been a definitive ruling, affecting all industries changed by automation, on how far unions can go to preserve traditional jobs.

The dissenters -- Chief Justice Warren Burger with Justices Potter Stewart, William Rehnquist and John Paul Stevens -- protested the court's unwillingness to confront that question.

The longshoremen's work rules were plainly designed as "nothing less than an invidious form of 'featherbedding' to block full implementation of modern technological progress," Burger wrote.

The work in question involves the loading and unloading of "land-based transportation -- the containers functioning as truck trailers -- away from the pier.

"The ILA demands that its members be paid for the utterly useless task of removing the contents and re-packing them or alternatively that a fine be imposed on" the shipping company. This merely provides "economically useless work" for ILA members, they said.