The Supreme Court, in a major defeat for organized labor, ruled yesterday that unions have no power to prevent members from defecting and crossing picket lines during a strike or when a strike seems imminent.

The 5-to-4 ruling, written by Justice Lewis F. Powell Jr., is a victory for the Reagan administration and the Reagan-dominated National Labor Relations Board, which had argued that federal labor law did not give unions the power to fine or discipline members in such situations.

The NLRB's interpretation of the federal Taft-Hartley Act of 1947 should be "accorded substantial deference," Powell said. " . . . the board was justified in concluding that by restricting the right of employes to resign, the union rule impairs the policy of voluntary unionism" that Powell said was "implicit" in the law.

A spokesman for the AFL-CIO said the court's ruling was a "serious" but not "crippling" blow to labor.

A spokesman for the U.S. Chamber of Commerce said the ruling "strikes at the heart of the economic power of unions -- the right to strike."

The case, Pattern Makers' League of North America, AFL-CIO v. NLRB, involved a small union in the Midwest that, like many unions, had an internal constitutional provision forbidding members from resigning during a strike or shortly before a strike. On May 5, 1977, two locals struck. The strike ended seven months later, but not before 10 employes had resigned from the union and returned to work.

The union refused to accept the resignations because they violated the no-resignation rule, and then levied fines on the members roughly equivalent to their earnings during the strike.

The NLRB in 1982 ruled that the fines were invalid; and the 7th U.S. Circuit Court of Appeals in 1983 upheld that ruling. The 9th U.S. Circuit Court of Appeals in California, however, reached the opposite conclusion in another case decided just two months after the 7th Circuit's rulng.

The administration urged the justices to reject labor's arguments. "Enforced solidarity is not the policy of our labor laws," the Justice Department told the high court.

The unions argued that Congress, in passing the Taft-Hartley Act, specifically rejected the position the administration favored.

Powell, joined by Chief Justice Warren E. Burger and Justices Byron R. White, William H. Rehnquist and Sandra Day O'Connor, said the legislative history cited by labor was "ambiguous" and said the NLRB's interpretation was "reasonable."

White, in a separate concurrence, said the law could be interpreted either way, and he would defer to the NLRB's interpretration if the board were urging the opposite result.

Justice Harry A. Blackmun, in dissent, accused the majority of "supinely" deferring to the NLRB's decision. The majority's "conclusion that freedom under the labor law means freedom to break a freely made promise to one's fellow workers after they have relied on that promise to their own detriment is not only a notion at odds with the structure and purpose of our labor law, but is an affront to the autonomy of the American worker."

Steve Bokat, general counsel for the U.S. Chamber of Commerce, said the ruling will probably enhance the power of business to encourage workers to return and gain agreement on a marginally acceptable offer that would be rejected had the union the power to sustain a lengthy strike.

The AFL-CIO, in a statement after the ruling, said that while the decision would "make it more difficult to conduct effective strikes, we believe that in the end the board's ultimate objective -- to cripple labor unions -- will not be achieved."

AFL-CIO spokesman Murray Seeger said the ruling will "encourage dissidents to push unions that have a no-resignation provision to take it out or to break ranks." Unions won't be able to "negotiate with the threat of an actual strike," he said.

In another important decision yesterday, the court enhanced the power of government agencies to pressure civil rights litigants to settle cases on terms more favorable to the government.

The 6-to-3 ruling written by Burger involved an apparent conflict between a federal court-imposed rule and the 1976 Civil Rights Attorneys' Fees Awards Act.

The act, passed to encourage people to defend their rights, provides that civil rights plaintiffs who win their cases are entitled to have the federal or state or local government pay their attorneys' fees. The fees often amount to more than the damages awarded.

Rule 68 of the Federal Rules of Civil Procedure, however, was designed to encourge settlements of lawsuits.

The rule, adopted by the federal Judicial Conference, was interpreted by a federal judge in Illinois to mean that if a plaintiff rejects a settlement offer and then receives a lower sum from a jury, he can not collect attorneys' fees incurred after the settlement offer.

In Marek v. Chesny, a suit by the father of a man killed by police, the jury awarded $60,000, less than the offered $100,000, and a judge declined to give the father $139,000 in legal fees incurred after the offer was made.

The federal rule was "neutral" and "simply encourages settlements," Burger said, upholding the trial judge's ruling.

Justice William J. Brennan Jr., joined by Justices Thurgood Marshall and Blackmun in dissent, said the rule favored defendants and the decision "inevitably will encourage defendants who know they have violated the law to make 'low-ball' offers and the result will put severe pressure on plaintiffs to settle."