The ruling on Thursday is the second decision in as many years narrowing the reach of federal environmental regulations. The court’s conservative majority boosted property rights over concerns about clean water.

The justices found in favour of a couple who sought to build a house near Priest Lake in Idaho’s panhandle. Chantell and Michael Sackett objected when federal officials identified a soggy portion of the property as a wetlands and required them to get a permit before building.

By a 5-4 vote, the court said wetlands may only be regulated if they have a “continuous surface connection” to larger, regulated bodies of water.

The court dropped the 17-year-old opinion by their former colleague Anthony Kennedy that allowed regulation of wetlands that have a “significant nexus” to larger waterways.

Kennedy’s opinion had been the standard for evaluating whether wetlands were covered under the 1972 Clean Water Act (CWA). Opponents had objected that the standard was vague and unworkable.

In the majority opinion issued with Thursday’s ruling, Justice Samuel Alito wrote that the US Environmental Protection Agency (EPA) was exceeding the powers granted under the CWA when regulating all wetlands.

While praising the CWA as a “great success” that led to the cleanup of severely polluted rivers and lakes, the conservative justice said its vague mandate has been an “unfortunate footnote”.

“The Act applies to ‘the waters of the United States’, but what does that phrase mean? Does the term encompass any backyard that is soggy enough for some minimum period of time?” Alito asked.

Environmental advocates predicted that narrowing the reach of that law would strip protections from more than half the wetlands in the country.

Reacting to the decision, Manish Bapna, the chief executive of the Natural Resources Defense Counsel, called on the US Congress to amend the CWA to restore wetlands protections and on states to strengthen their own laws.

“The Supreme Court ripped the heart out of the law we depend on to protect American waters and wetlands,” Bapna said in a statement. “The majority chose to protect polluters at the expense of healthy wetlands and waterways. This decision will cause incalculable harm. Communities across the country will pay the price.”

At the White House, Press Secretary Karine Jean-Pierre said the decision “aims to take our country backwards. It will jeopardise the sources of clean drinking water for farmers, businesses and millions of Americans.”

Jean-Pierre said President Joe Biden will “use every legal authority available to him” to ensure Americans have clean drinking water.

The outcome almost certainly will affect ongoing court battles over new wetlands regulations that the Biden administration put in place in December. Two federal judges have temporarily blocked those rules from being enforced in 26 states.

Scientists say protecting wetlands, which naturally capture planet-warming emissions, is key to combating climate change.

But in Thursday’s ruling, all nine justices agreed that the wetlands on the Sacketts’ property are not covered by the act. Yet only five justices joined in the majority opinion, imposing a new test for evaluating when wetlands are covered by the CWA.

Conservative Brett Kavanaugh and the court’s three liberal justices charged that their colleagues had rewritten the law with their opinion.

“The Court’s erroneous test not only will create real-world consequences for the waters of the United States, but also is sufficiently novel and vague (at least as a single standalone test) that it may create regulatory uncertainty for the Federal Government, the States, and regulated parties,” Kavanaugh wrote.

Justice Elena Kagan wrote that the majority’s rewriting of the act was “an effort to cabin the anti-pollution actions Congress thought appropriate”. Kagan referenced last year’s decision limiting the regulation of greenhouse gas emissions under the CWA.

In both cases, she noted, the court had appointed “itself as the national decision-maker on environmental policy”. Kagan was joined in her opinion by her liberal colleagues Sonia Sotomayor and Ketanji Brown Jackson.

Thursday’s decision is part of an ongoing trend. Since former President Donald Trump appointed the last of his three Supreme Court justices in 2020, the high court has had a solid conservative majority, allowing for the rollback of regulations and the advancement of right-wing priorities.

Those priorities included overturning the constitutional right to abortion, which the court ordered last year.