The conservative supreme court justice Clarence Thomas failed to disclose more private travel on a jet owned by the rightwing mega-donor Harlan Crow, a Democratic senator said on Monday, amid a swirling ethics scandal and demands for judiciary reform.

“I am deeply concerned that Mr Crow may have been showering a public official with extravagant gifts, then writing off those gifts to lower his tax bill,” Ron Wyden of Oregon, the Senate finance committee chair, told a lawyer for Crow in a letter.

“This concern is only heightened by the committee’s recent discovery of additional undisclosed international travel on Mr Crow’s private jet by Justice Thomas.”

The committee, Wyden said, had “obtained international flight records showing that on 19 November 2010, Justice Thomas and his wife [the rightwing activist Ginni Thomas] flew from Hawaii to New Zealand on Mr Crow’s private jet, before flying back from New Zealand to Hawaii on the jet a week later on 27 November 2010. Mr Crow was also a passenger on these flights.

“To date, Justice Thomas has never disclosed this private jet travel on any financial disclosure forms, even though Justice Thomas has amended disclosures to reflect other international travel on Mr Crow’s private jet.”

Thomas was confirmed in 1991, after stormy hearings in which he was accused of sexual harassment by Anita Hill, a law professor. More than 30 years later, he is the longest-serving rightwinger on a court controlled by Republican nominees, 6-3.

Gifts to Thomas, undeclared and primarily from Crow but also from other sources and to other justices, have stoked an ethics crisis. Apparent political sympathies among the judges – and family members, with Ginni Thomashaving participated in Donald Trump’s attempt to overturn the 2020 election – have only added fuel to the fire.

The non-profit newsroom ProPublica won a Pulitzer prize for its reporting on the issue. In Congress, the New York progressive Alexandria Ocasio-Cortez has led calls for Thomas and Samuel Alito, his fellow arch-rightwinger, to be impeached and removed – a political gambit without a realistic chance of success.

Supreme court justices are nominally subject to the same ethics rules as all federal judges but in practice police themselves. An ethics code was introduced last November but remains without outside means of enforcement.

Elena Kagan, one of three liberals on the court, has called for better enforcement of ethics rules. John Roberts, the rightwing chief justice, apparently remains unmoved. Despite calls from Wyden and other Democrats, Roberts has refused to testify in Congress.

All the while, the Roberts court has handed down major conservative victories, including removing the federal right to abortion and saying presidents enjoy immunity for some acts.

Joe Biden recently introduced reform proposals including term limits. In response to the president’s proposal, Neil Gorsuch, the first of three rightwingers appointed in four years during Trump’s presidency, used an interview with Fox News to warn: “I just say: be careful.”

Thomas has said he initially believed he did not have to disclose gifts from donors. In the case of Crow, a real-estate billionaire known for collecting memorabilia associated with Nazi leader Adolf Hitler, gifts to Thomas also included resort stays, a property purchase and payment of school fees.

Thomas did not immediately comment about Wyden’s letter.

As reported by the New York Times, a spokesperson for Crow said his lawyers had “already addressed Senator Wyden’s inquiries, which have no legal basis and are only intended to harass a private citizen”.

Crow “consider[s] this matter settled”, they said.

Wyden said he was “concerned that I have so far been unable to even determine the full extent of the potential tax abuse at issue”.

A committee spokesperson told the Times the panel still hoped Crow would provide tax records voluntarily.

Wyden said: “Neither Mr Crow nor Justice Thomas have disclosed the full scale of the Thomases’ use of the Michaela Rose [Crow’s superyacht] and private jets courtesy of Mr Crow, even as the Congress continues to uncover additional international private jet travel with Mr Crow that Justice Thomas failed to disclose on his ethics filings.”

Citing filings recently updated by Thomas, “to include an eight-day voyage aboard the Michaela Rose in Indonesia in 2019”, Wyden said the justice “still has not disclosed other trips” on Crow’s yacht”.

He added: “Public reports show evidence that Justice Thomas was a passenger aboard the Michaela Rose in Greece, New Zealand and elsewhere.

“Additionally, a relative of Justice Thomas has stated that he personally witnessed Justice Thomas travel aboard the Michaela Rose in the Caribbean, Russia and the Baltics, with the trip to Russia also including helicopter ride(s).”

Caroline Ciccone, president of Accountable.US, a group which campaigns for court reform, said: “The undue influence exerted by wealthy and powerful individuals like Harlan Crow over Justice Thomas highlights systemic corruption that cannot be ignored.”

Andrew Bates, a White House spokesperson, said Wyden had “strengthen[ed] the case [the president] made for common-sense reforms that are backed by constitutional experts across the political spectrum – as well as the vast majority of the American people.

“The most powerful court in the United States shouldn’t be subject to the lowest ethical standards, and conflicts of interest on the supreme court cannot go unchecked.”