Supreme court justice Samuel Alito, who owns stock in oil companies, may be violating court ethics codes by participating in certain cases that could benefit big oil, government watchdog groups say.

In a Thursday letter, a coalition of watchdog organizations called on the Senate judiciary committee to investigate Alito, the sole supreme court justice with holdings in energy companies.

“His irregular recusal practice in oil and gas industry-related cases is undermining public confidence in the impartiality of the Court,” says the letter, signed by green groups including League of Conservation Voters and Center for Biological Diversity, as well as progressive accountability watchdogs the Revolving Door Project and True North Research.

The high court in February agreed to take up a case brought by oil majors Suncor Energy and Exxon – the first time the court agreed to weigh in on such a challenge. The companies asked the justices to find that federal law prevents subnational governments from filing lawsuits against oil and gas companies for the climate-warming effects of their products.

The court did not say which justices supported weighing in on the petition. Alito did not recuse himself, the letter notes.

“No judge on any court, including the high court, should be allowed to hear cases where he or she have a financial stake in those cases,” said Lisa Graves, a former senior justice department official who now directs True North Research.

In 2023, Alito recused himself from considering a petition brought by the same companies in the same lawsuit. That request, which would have required approval from four judges, was denied.

The justice’s most recent financial disclosure, which was filed last August and covers 2024, showed holdings in individual stock worth between $60,007 and $245,000 in ConocoPhillips, Phillips66 and five other oil and energy companies. Alito also has up to $100,000 invested in a Vanguard fund in which Exxon is the third-largest holding, the letter says.

“These holdings alone should compel Justice Alito to recuse himself from the Boulder case and the parallel state climate deception cases,” the groups say, referring to lawsuits brought by more than 70 state and local governments accusing oil companies of misleading the public about their role in the climate crisis.

It is not clear if Alito has sold his stock in oil and gas companies since filing his last financial disclosure. The Guardian has contacted the supreme court and Alito for comment.

Justices will be required to report on their 2026 holdings next year; by then, the court may have already ruled on the Suncor case, Graves said.

The groups say Alito has another “apparent conflict of interest” – his relationship with Republican billionaire donor Paul Singer. Singer founded and runs the hedge fund Elliott Investment Management, which owns more than 52m shares of Suncor which are worth more than $2.3bn.

ProPublica reported in June 2023 that Alito failed to officially disclose that he took a private jet ride to Alaska for a 2008 fishing trip paid for by Singer. Alito defended the trip in the Wall Street Journal, saying ethics rules didn’t require him to disclose that he took the trip and that he had no duty to recuse himself from any cases involving Singer discussed in the reporting. He wrote: “ProPublica suggests that my failure to recuse in these cases created an appearance of impropriety, but that is incorrect.”

Thursday’s letter from the watchdog groups says: “Alito’s decision to reverse course and participate in granting the companies’ most recent petition – when a finding in favor of the companies could directly and indirectly benefit both himself and his billionaire friend – is an indefensible breach of ethical boundaries.”

In 2023, the supreme court adopted its first-ever formal ethics code – a response to pressure over a slew of scandals focused on some of its senior rightwing justices. It says justices should recuse themselves from cases where their “impartiality might reasonably be questioned”, but allows the justices to make that decision themselves.

The code has been widely derided as toothless by experts due to its lack of an enforcement mechanism. Unlike standards for other federal judges, it also allows justices to stay on cases if their vote is necessary to resolve the case.

“It’s possible that Alito is using that rationale, [arguing] he’s needed to resolve the matter of the Suncor case,” said Graves. “It’s really outrageous. The highest court in the country … should have the highest standards, not the lowest ones.”

This year, the high court also rolled out new software to scan challengers’ filings to identify potential conflicts of interest which might require justices to recuse themselves from cases. Parties before the court must list stock-ticker symbols for companies involved in cases to allow the new software to help identify conflicts.

But the outcome of each climate accountability lawsuit targeting big oil could affect the entire industry, said Hannah Story Brown, deputy research director at Revolving Door Project. That means holdings in any oil companies should disqualify justices from weighing in on any of the lawsuits, she said.

“A blanket refusal is the only consistently ethical option for Alito when faced with any of these parallel cases,” Brown said.