The Environmental Protection Agency has decided not to limit perchlorate, a chemical that has long been detected in the drinking water of many Americans and linked to potential brain damage in fetuses and newborns and thyroid problems in adults, according to two agency officials briefed on the matter.

They spoke on the condition of anonymity because the decision hasn’t been announced.

The move, which comes despite the fact that the EPA faces a court order to establish a national standard for the chemical compound by the end of June, marks the latest shift in a long-running fight over whether to curb the chemical used in rocket fuel.

Under President Barack Obama, the EPA had announced in 2011 that it planned to set the first enforceable limits on perchlorate because of its potential health impacts. Both the Defense Department and military manufacturers have long resisted any restrictions on the chemical, which is also used in fireworks, munitions and other ignition devices. It naturally occurs in some areas, such as parts of the Southwest.

In an email Thursday, EPA spokeswoman Corry Schiermeyer said the agency “has not yet made a final decision” on whether to limit perchlorate in drinking water. “The next step in the process is to send the final action to the Office of Management and Budget for interagency review,” she said. “The agency expects to complete this step shortly.”

The New York Times first reported the agency’s decision.

The EPA also issued a news release Thursday in which Administrator Andrew Wheeler hailed the fact that levels of perchlorate exposure have declined since 2011. Though no federal standards regulating perchlorate levels in drinking water exist, some states have already acted to reduce the amounts in their drinking water systems. California and Massachusetts, for example, have set limits for perchlorate at levels far lower than what the EPA had previously proposed.

“Because of steps that EPA, states and public water systems have taken to identify, monitor and mitigate perchlorate, the levels have decreased in drinking water,” Wheeler said. “This success demonstrates that EPA and states are working together to lead the world in providing safe drinking water to all Americans.”

Environmental advocates were quick to criticize the EPA, saying the failure to institute a national limit on perchlorate in drinking water will leave many Americans vulnerable to potentially harmful health effects.

“It’s a real slap in the face of science, as well as to the court order and the law,” Erik Olson, a water expert at the Natural Resources Defense Council, said in an interview. “It’s a bad precedent on so many levels.”

In a separate blog post on Thursday, Olson said failing to regulate the compound would amount to “a deeply disturbing violation of the agency’s mission.”

Some groups, however, have urged the EPA not to set a federal threshold for perchlorate, saying existing evidence does not warrant it. For instance, in comments last year, both the American Chemistry Council and the American Water Works Association recommended that the EPA withdraw the 2011 determination to impose a national standard.

G. Tracy Mehan III, executive director of government affairs for the water works association, wrote that regulating perchlorate would not present a “meaningful opportunity” to reduce health risks, and that the benefits of such regulation would not justify the costs. “If EPA proceeds,” Mehan wrote, “it will set a troubling precedent and undermine the scientific credibility of the Agency’s regulatory process under the Safe Drinking Water Act.”

Last summer, the EPA sought input on a range of possible limits it was considering on perchlorate in drinking water. The one the agency appeared to favor at the time was a standard of 56 parts per billion — a threshold that public health officials called far too weak, and one that was several times more lenient than the EPA itself had set in a 2009 health advisory.

Even as it sought input on possible regulation last summer, the EPA left open the possibility that it would walk away from the matter, particularly if it determined that the chemical did not occur at levels deemed to present a serious public health risk.

Some health experts pleaded with the agency not to take that approach, including Kyle Yasuda, then-president of the American Academy of Pediatrics. In a letter to the EPA, Yasuda in August urged the agency to adopt the strongest possible curbs on the chemical, based on the “well-established harms of perchlorate ingestion for children.”

“AAP is particularly concerned that EPA is considering withdrawing its 2011 determination to regulate perchlorate, relinquishing national oversight over a chemical with well-established health risks in drinking water,” Yasuda wrote. “This would set a precedent inconsistent with EPA’s stated mission to protect public health.”

Though the EPA has set legal limits on more than 90 contaminants in drinking water, including lead, arsenic and mercury, a far broader universe of “emerging contaminants” remains unregulated.

The agency has long kept tab on scores of substances that have surfaced in water systems around the country, with the aim of restricting those that endanger public health. But partly because the rules the agency must follow are complicated and contentious, officials have yet to limit any new contaminant for decades. Perchlorate is the only chemical to come close to regulation since the 1990s. Time and again, regulators have backed away.

The last time came on a Friday in 2008, when the Bush administration formally declined to set a drinking-water safety standard for perchlorate. With little fanfare, the agency issued a news release saying it had “conducted extensive review of scientific data related to the health effects of exposure to perchlorate from drinking water and other sources and found that in more than 99 percent of public drinking water systems, perchlorate was not at levels of public health concern.”

In that instance, according to documents obtained by The Washington Post at the time, White House officials heavily edited the scientific findings in the EPA’s rulemaking documentation.