Mayor Marion Barry formally endorsed a bill yesterday that would roll back to $396 a week the maximum disability payment to injured D.C. workers and give broad new powers to employers who challenge injury claims.

Key business and labor officials said they were dissatisfied with Barry's proposals after they emerged from separate private briefings with the mayor. The proposals were in the form of 47 amendments to a bill introduced by City Council member Wille J. Hardy (D-Ward 7).

Barry's recommendations came on the eve of what is expected to be a vigorous City Council debate tonight at 7:30 over Hardy's bill. The action will be in the council chambers. A spokesman for Barry said he was considering extra security measures to control hundreds of anticipated advocates and spectators.

Among the mayor's chief recommendations:

Employers would no longer be required to provide "substantial evidence" that a worker's injury claim was not valid. This key victory for business would allow an administrative judge to put a greater burden on an injured worker to rebut his boss' evidence that his injury was not job-related.

Injured workers would not be required to seek treatment from a panel of doctors selected by their employer. The mayor proposes to select a citywide panel of doctors, but many details have not been worked out. Workers had feared that employer-selected physicians would not be impartial.

Employers who contest workers' injury claims unsuccessfully will be required to pay the attorney's fees of the worker.

Workers' benefits will increase yearly but not more than 5 percent. A counter-proposal more favored by labor had sought a ceiling of 9 percent.

Barry revealed in the private briefing sessions that his corporation counsel, Judith Rogers, has issued a new opinion reversing an earlier conclusion by her deputy that the District of Columbia does not have the authority to take over administration of the disability payment program.

The program, called workers' compensation, now is administered by the U.S. Department of Labor under a 1928 law. Federal labor officials have said that if the D.C. City Council passes a law taking over the system, the federal government effectively will ignore it.

"If they pass the bill, nothing happens unless Congress acts," said Donald Elisburg, assistant U.S. labor secretary. "We have no objection per se to D.C. having its own (program). The question is how do you do it?"

Greater Washington Central Labor Council President Robert E. Petersen said at a press conference yesterday afternoon that his members -- and a coalition of religious and consumer groups -- will challenge the legislation in federal court if the council should pass it.

At issue are millions of dollars in worker injury benefits paid to nongovernment D.C. workers. Private employers are required by law to carry insurance against job-related injuries. In return, workers give up the right to sue employers for negligence in accidents.

Workers' injury insurance was a virtually unnoticeable expense in the city until 1972, when Congress liberalized the benefits (then a maximum of $70 a week) to keep up with inflation. Today, the maximum payment for total disability is two-thirds of a workers' salary tax-free up to $426 a week.

Private insurance companies that sell workers' compensation coverage to employers have reacted to growing benefits by seeking substantial increases in their premiums. District of Columbia employers paid about $20 million in premiums in 1972, but under the new system premium costs have soared to a projected $140 million this year.

The Washington Post, one of the largest private employers in the city, is projecting its workers' compensation premium from Liberty Mutual Insurance Co. to reach $7 million in 1980, compared with a 1972 cost of about $175,000.

Labor council official Ron Richardson suggested during the press conference that The Post editorial page support of workers' compensation reform stemmed from a "vested interest." He said The Post "should remain silent because it is not in the best interest of its readers or subscribers."

Post President Thomas H. Ferguson said yesterday that he was meeting privately with City Council members to gain support for the company's position. He said he emphasized that he was representing the business side of the newspaper and not the editorial department.

"My only reason for sharing our concerns with members of the council is that the workers' compensation costs to this company are close to being unbearable," Ferguson said.

"If the Hardy bill is voted down," he said, "we'll have to think seriously about moving The Post -- I'm not saying we'll do it, but we'll think about it very, very seriously." Even though both business and labor groups were not happy with Barry's recommendations, business officials clearly were not smarting as badly as labor officials.

"The mayor was somewhat less 'harsh than Hardy,' said private lawyer Mark Schaffer, whose firm specializes in workers' compensation cases. "But where she (Hardy) would have put us back 30 years, the mayor is putting us back 20." s

Roger L. Blunt, president of Tyroc Construction Co. in Northeast Washington and chairman of the Greater Washingotn Board of Trade committee that met with Barry during the afternoon, said of the recommendations, "They don't go far enough, but it is movement toward the reform that we need."

Board of Trade executive vice president John Tydings said that if the council passes the Hardy bill, "no one doubts (there will be) a legal challenge."

He criticized labor officials for refusing to compromise on workers' compensation reform. The labor council has said that it prefers the system as it is.