Newspapers and broadcasters lost what they considered to be a major legal battle yesterday when the Supreme Court made them more vulnerable to libel suits brought by private consultants to government.

The court, acting on a suit against the Loudoun Times-Mirror in Virginia, let stand an appeals court ruling that such consultants are not public figures or public officials, who must meet the difficult requirement of proving media malice in order to win a libel action. That requirement, which does not apply to private parties, often insulates reporting from costly libel trials and awards.

The Times-Mirror and the industry as a whole had argued that the 4th U.S. Circuit Court of Appeals ruling would stifle reporting on consultants, whose ever-expanding role in government has produced an increasing number of critical news reports and exposes.

The suit against the Times-Mirror stemmed from an article in September 1978 concerning the Irgoquois Research Institute, a private consulting firm hired by the Fairfax County Water Authority to study the impact of the construction of water intake facilities on Lowes Island in the Potomac River. The article quoted an amateur archeologist as questioning the quality of the institute's work. The archeologist said his comments were taken out of context, and the consultant sued the paper, the reporter and the archeologist for $200,000 each.

U.S. District Court Judge Albert Bryan threw the case out before trial, saying the consultant firm was a "puplic official" because of its work for the government, and that there was no evidence of malice on the part of the newspaper. The appeals court, in an opinion written by judge James M. Sprouse, disagreed and sent the case back for trial, which may now proceed becuase of the Supreme Court's action yesterday.

The 4th Circuit ruling applies only in Maryland, Virginia, West Virgina and North and South Carolina. But the Reporters Committee for Freedom of the Press, the American Newspaper Publishers Association and other industry groups said in an anicus curiae brief that they feared it could influence other rulings and have "far-reaching ramifications" that could inhibit criticism of governmental affairs."

In other action yesterday:

The court agreed to review the law prohibiting the use of residential mailboxes for anything but mail. Civic groups in Greenburgh, N.Y., successfully challenged the law in the lower courts because it prevented them from distributing their messages in mailboxes. A U.S. District Court judge ruled (in U.S. Postal Service vs. Council of Greenburgh Civic Associations) that the restriction violated the First Amendment right to free expression.

The justices agreed to consider a business' right to go out of business without negotiating with its employes. The National Labor Relations Board had held that an employer who shut down part of his building maintenance firm violated labor laws by not discussing the decision with the union involved. The U.S. Chamber of Commerce, which urged the court to take the case of First National Maintenance Corp. vs. NLRB, has attacked the NLRB finding as an intrusion on a purely management shphere of decision-making.

The court let stand a citizen challenge to the cleanup at Pennsylvania's Three Mile Island nuclear power plant. Four Lancaster County residents filed suit after the 1979 accident because of fears that water used in the cleanup would be discharged into the Susquehanna River. The plant's owner contended that the citizens had to take their complaints to the Nuclear Regulatory Commission before going to the courts. The 3rd U.S. Circuit Court of Appeals ruled for the citizens, who hailed the ruling as creating a private method to enforce environmental laws in the face of bureaucratic delay or recalcitrance. Justice William Rehnquist wrote a dissent to the Supreme Court action yesterday and was joined by Chief Justice Warren Burger and Justice Lewis Powell.