Washington (AFP) - A billionaire Republican donor paid for the private school tuition of Justice Clarence Thomas's grandnephew, whom he was raising, a report said Thursday -- the latest in a string of ethics controversies surrounding the US Supreme Court.

Thomas did not disclose the payments, according to ProPublica, which also reported last month that real estate tycoon Harlan Crow had gifted the longest-serving justice luxury trips over two decades.

A July 2009 bank statement showed that Crow's business had paid a $6,000 monthly bill for a boarding school attended by Mark Martin, Thomas's teenage grandnephew, the report said.

Thomas had legal custody of Martin.

A former school administrator told the news outlet that Crow paid for the year Martin spent at Hidden Lake Academy in the state of Georgia, and that Crow once told him he also paid Martin's fees at another school.

If he paid for all four years at the two schools, Crow could have spent more than $150,000 in tuition, ProPublica said.

But Thomas, 74, did not disclose the payments, despite records showing he did report another friend's $5,000 education gift for Martin. 

Ethics standards for the nine justices, who serve lifetime terms, have been under increased scrutiny after reports of lavish gifts and undisclosed real estate transactions.

Neither Crow's company nor staunch conservative Thomas immediately responded to AFP's queries about the arrangement.

Crow's office told ProPublica that he "has long been passionate about the importance of quality education and giving back to those less fortunate, especially at-risk youth."

A man who called himself Thomas's friend, and who has worked as the justice's wife Ginni Thomas's lawyer, tweeted Thursday that Crow had paid for two years of Martin's private education.

Mark Paoletta said the Thomases "devoted twelve years of their lives to taking in and caring for a beloved child." 

"Their friends joined them in doing everything possible to give this child a future," he added, saying ethics reporting rules do "not include a 'great nephew'" in the definition of "dependent child" for whom gifts must be disclosed.

"It is limited to a 'son, daughter, stepson or stepdaughter,'" he added.

Ethics questions

Senator Dick Durbin on Thursday repeated his call for a binding ethics code for Supreme Court justices, saying in a statement the body's "reputation and credibility" were at stake.

"The highest court in the land should not have the lowest ethical standards," added Durbin, chair of the Senate Judiciary Committee.

Earlier this week, the committee voiced dismay after Chief Justice John Roberts declined to testify before the panel amid the mushrooming controversy.

Previous ProPublica investigations revealed Thomas also failed to disclose yacht and private plane trips gifted by Crow, and that Crow bought properties from the justice.

Crow has made more than $10 million in donations to Republican political groups, ProPublica said, including half a million dollars to a conservative lobbying group founded by Ginni Thomas, who has drawn her own criticism for supporting Donald Trump's attempt to overturn his 2020 presidential election loss.