Bowing to pressure from coal companies, the Department of Labor has postponed a requirement that would equip all underground miners with a new life-saving self-rescue device.

An order proposed in 1978 by Labor's Mine Safety and Health Administration to put the oxygen-generating units in underground mines was to have taken effect this month.

But MSHA administrator Robert B. Lagather, reacting to industry complaints that the devices were inadequately tested, postponed the effective date for six more months, to June 21. MSHA tests will continue.

About 125,000 underground coal miners will be affected by the ruling. At least 40,000 more metal and non-metallic miners will be covered later by the rugulation, an MSHA spokesman said.

Lagather's decision was announced in the Federal Register on an emergency basis, which precluded the customary process of hearing public comment before changing the proposed effective date.

The move drew a bitter rebuke from the executive board of the United Mine Workers, and a public-interest law firm representing miners has petitioned the U.S. Cricuit Court of Appeals for a stay of the MSHA order. J. Davitt McAteer, an attorney for the Center for Law and Social Policy, charged that failure to put the new devices into the mines at once would needlessly imperil miners.

Two industry trade groups had requested the delay for most testing in a letter to MSHA last month.

McAteer said that telephone logs obtained from MSHA indicated that Lagather had received calls from top coal executives who oppose the devices just one day before his order was send to the Federal Register. Lagather could not be reached for comment.

The battle over the self-rescuers, required by the 1969 Federal Coal Mine Health and Safety Act, has gone on for a decade and cost the government hundreds of thousands of dollars in research and regulatory activity.

The Bituminous Coal Operators Association and the American Mining Congress have opposed the mine-safety regulators at nearly every turn, charging that the devices were too costly, not reliable or inadequately tested. c

The new device, expected to cost $375, will replace a less effective unit now selling for about $75.

The current unit only converts lethal carbon monoxide to carbon dioxide, while filtering out no other toxic fumes, and lasts for no more than one hour.

MSHA's new chemical units create a "closed" protective system of oxygen generation that guarantees the miner a one-hour supply under duress or movement and four to six hours if he is in repose.

The devices are designed to protect miners in the critical period after an accident while rescuers go to their assitance.

The new units, if they turn out to be as effective as MSHA believes, could have a dramatic life-saving impact in underground fires and other types of accidents in which air supplies are cut off or poisoned.

A 1968 study by the National Academy of Science showed that roughly 17 percent of underground mining fatalities could be averted if better self-rescue devices were available for individual miners. Industry-sponsored studies reject those findings.

In revising mine-safety law in 1969, Congress required that better units be made available. Victims of a number of major accidents since then could have been saved with better self-rescuers, MSHA officials say.

UMW safety director Everett Acord said the union believes the new oxygen units are sufficiently reliable to be put into use immediately. He said one of the devices was used successfully in rescue work at a recent West Virginia mine disaster.

"I find it difficult to believe that anyone could ask for a stay of the regulations when it is well known that the self-contained self-rescuer is a safe and reliable unit," Acord said. Any delay in installing the devices "would border on criminal negligence," he said.