The federal courts, reversing a ban in effect for decades, will allow television cameras into civil trials, under a decision reached yesterday by the Judicial Conference of the United States.

The decision culminates a seven-year campaign by the news media to open up the federal courts, one of the last holdouts against televising trial proceedings. Forty-five states, including Maryland and Virginia, allow cameras in their state courts under a variety of programs.

The new federal program could for the first time permit cameras in a courtroom in the District, which, along with five states, still bans their use in the local courts.

Proponents hailed the decision as a victory for both the public and the court system. "One would hope this is a first step in having cameras generally in the federal courts . . . and that would result in a quantum leap in the public understanding of the system," said Timothy B. Dyk, a D.C. lawyer who represents a consortium of news organizations pressing for the change.

The plan, a three-year experimental program in a small group of courts, was "overwhelmingly approved" by a voice vote of the Judicial Conference, which sets policy for the federal courts, according to spokesman David Sellers.

Sessions of the conference are closed, but Sellers said the 26 judges, presided over by Chief Justice William H. Rehnquist, debated the proposal, with "three or four" objecting, some on the ground that the cameras might cause "grandstanding" by judges and lawyers.

Federal judges have been under increasing pressure to reverse the prohibition against cameras that was instituted in 1946 for criminal trials and broadened to all proceedings in 1962. Sellers said the judges believed that the introduction of cameras to their courtrooms was inevitable, and preferred to "conduct this experiment rather than have this forced upon them," possibly by Congress.

Rep. Robert W. Kastenmeier (D-Wis.), chairman of the House Judiciary subcommittee that oversees federal courts, had called on the conference to lift the ban, and said yesterday that viewing trials firsthand would foster more public trust in the court system.

News organizations, including three television networks, some of the nation's largest newspapers and C-SPAN, a public affairs cable network, renewed their quest for the change in 1988, and the televising of court proceedings is now becoming a big business.

New York publisher Steven Brill and Time Warner Inc. are about to begin a 24-hour cable network that will televise trials and legal commentary. Brill lobbied for the change, appearing before a special committee that recommended last month that the conference approve this program.

Brill peppered the committee with favorable reports about cameras in the courts, presented a demonstration tape of trial coverage and argued that none of the states that have experimented with cameras has found significant difficulties.

Brill predicted that federal judges would decide "this isn't such a bad thing . . . once they know more about what we do."

The voluntary program will begin next July, and will permit -- but not require -- cameras in one or two appeals courts and in trial courts in up to six federal districts around the nation. The experiment applies only to civil cases, and television coverage of federal criminal trials remains prohibited.

Judges will need to volunteer to be part of the experiment, and it is not yet clear what courts will join.

Judges will retain broad powers to reject any request for coverage, and coverage will be regulated down to the tiniest detail, including a requirement that camera operators "shall wear appropriate business attire."

Prohibitions against cameras were spurred by the film and radio coverage of the 1935 Lindbergh kidnapping trial, which turned a New Jersey courtroom into a circus. The American Bar Association passed a canon of ethics barring radio and film coverage, which was then adopted by state courts in most jurisdictions and the federal courts, according to Dyk.

But in 1977, Florida broke from the pack and passed rules allowing TV coverage of trials. Since then, 44 other states have instituted either experimental or permanent programs to permit some type of TV coverage.

Still, the issue remains controversial, and many prosecutors and defense lawyers oppose camera coverage of criminal trials, arguing that it may adversely affect witnesses and jurors, and impede the ability to get at the truth.

"Whatever the marginal benefits, they are outweighed by the potential risks to the truth-finding process," said D.C. lawyer Seth Waxman, a staunch opponent.

U.S. District Judge James C. Cacheris, of Alexandria, a member of the special committee that proposed the change, but an opponent of cameras, said the mail he received showed that "an overwhelming majority of federal judges are opposed to it."

The Supreme Court, which does not allow the use of cameras there, is not affected by the conference's action.