The Supreme Court curtailed the power of federal agencies to implement laws passed by Congress, in a decision Friday that overturned a 40-year precedent that required judges to defer to agency interpretations of ambiguous laws.

The 6-3 opinion overturned what’s known as the Chevron doctrine, which federal agencies asserted regularly when defending their regulations in court, including the Justice Department, EPA and the Federal Communications Commission.

While the specific decision in this case throws out a federal fishery inspection rule, it throws open a new chapter in the balance of power between the three branches of the federal government.

Experts have said the overturning of Chevron could mean more conflicting decisions across the country on regulatory challenges, and such a decision would require Congress to staff up to write statutes with a level of specificity that forecloses the legal ambiguities that provide openings for lawsuits.

The decision also sides with Republican lawmakers and the conservative legal movement who have been critical of the Chevron doctrine for years, describing it as a way that regulatory agencies go beyond what Congress intended when it passed laws.

In a majority opinion, Chief Justice John G. Roberts Jr. wrote that instead of assuming that Congress meant to let agencies work out ambiguities in federal law, the courts are meant to decide any confusion.

“The better presumption is therefore that Congress expects courts to do their ordinary job of interpreting statutes, with due respect for the views of the Executive Branch,” Roberts wrote. “And to the extent that Congress and the Executive Branch may disagree with how the courts have performed that job in a particular case, they are of course always free to act by revising the statute.”

Roberts wrote that following Friday’s decision, courts may give “careful attention” — but not defer to — the executive branch agency defending a rule and only defer to an agency when Congress explicitly said an agency can make its own decision.

Justices on the liberal wing of the court, in a dissent written by Justice Elena Kagan, criticized the majority for elevating the power of the judicial branch over the executive and Congress, echoing past criticisms of the conservative majority’s administrative agency decisions.

“A longstanding precedent at the crux of administrative governance thus falls victim to a bald assertion of judicial authority. The majority disdains restraint, and grasps for power,” Kagan said.

The dissent accused the majority of appointing itself the “czar” of the federal government, with the ability to weigh in on any issue that the court wishes.

Reading from her dissent on the bench, Kagan said that the Chevron doctrine has been a foundation of American law for decades.

“It has become part of the warp and woof of modern government, supporting regulatory efforts of all kinds — to name a few, keeping air and water clean, food and drugs safe, and financial markets honest,” Kagan wrote.

The sweeping change came as a result of two cases, Loper Bright Enterprises v. Raimondo and Relentless, Inc. v. Department of Commerce, challenging the legality of a fishery inspection rule that required boats to pay the cost of inspectors. Roberts’ ruling in the Loper Bright case covered both cases. Jackson recused herself from the Loper Bright case.

They used those cases to ask the justices to overturn Chevron, after federal appeals courts for the District of Columbia and 1st Circuit relied on the doctrine to uphold a fishery inspection rule. Justice Ketanji Brown Jackson recused herself from the Loper Bright case because of her previous posting at the D.C. Circuit, and did not participate in that case.

The Chevron doctrine stems from a 1984 Supreme Court case, Chevron USA, Inc. v. Natural Resources Defense Council, Inc., that upheld a Reagan-administration decision by the EPA led by Anne Gorsuch, the mother of Justice Neil M. Gorsuch.

Since then it has become a major target of the conservative legal movement, who argued it gave too much power to unelected bureaucrats. Legislation to overturn passed the House in 2017, but that measure did not advance in the closely divided Senate.

Friday’s decision is the latest in a series of cases where the current conservative controlled court has restricted the the reach of administrative agencies, including a new “major questions doctrine,” which requires Congress to explicitly address “major questions of economic or political significance.”

In prior years the court used that doctrine to jettison the Biden administration’s student loan forgiveness plan and the EPA’s effort to regulate greenhouse gas emissions.