Paul Kamenar, attorney for Andrew Miller, joined by Peter Flaherty (left), chairman of the National Legal and Policy Center, talked to reporters after a federal judge found Miller in contempt for refusing to testify before the grand jury in August. J. Scott Applewhite/Associated Press/Associated Press

WASHINGTON — A federal appeals court on Tuesday upheld the appointment of Robert Mueller, the special counsel investigating ties between the Trump campaign and Russia, and ruled that a former aide to Roger J. Stone Jr., the longtime Trump adviser who was indicted last month, must testify before a grand jury.

In a unanimous ruling, a three-judge panel on the US Court of Appeals for the District of Columbia Circuit rejected a challenge to Mueller’s authority brought by Stone’s aide, Andrew Miller, who has been resisting a subpoena since last summer. After a US District Court judge held him in contempt of court, he appealed.

But in a 16-page opinion, Judge Judith W. Rogers upheld Mueller’s authority, holding that “Miller’s challenge to the appointment of the Special Counsel fails. Accordingly, we affirm the order finding Miller in civil contempt.”

Rogers, an appointee of President Bill Clinton, was joined by Judges Karen Henderson and Sri Srinivasan, who were appointees of Presidents George Bush and Barack Obama, respectively.

Paul D. Kamenar, an attorney for Miller, said they would probably appeal, although they had not yet decided whether to ask the full appeals court to review the panel’s decision or whether to take the matter directly to the Supreme Court.

“We are disappointed with the decision and will be considering future legal action whether before the full court of appeals or the Supreme Court,” he said in a phone interview.

Essentially, Miller’s legal team had argued that Mueller wielded too much power to qualify as an “inferior officer” under the Constitution, the type who can be appointed without Senate confirmation. Mueller was appointed in that manner when the deputy attorney general, Rod J. Rosenstein, named him as special counsel in May 2017. If Mueller was improperly appointed, they said, the subpoena was invalid.

But at a hearing over the dispute before the appeals court panel in November, Michael R. Dreeben, one of the lawyers working for Mueller, argued that Mueller was not “off in a free-floating environment,” but rather was subject to the supervision and control of the attorney general. As a result, he maintained, there was no need for Mueller to have been confirmed by the Senate. The appeals court agreed.

Miller’s attempt to challenge Mueller’s authority was similar to an earlier challenge brought by Paul Manafort, President Donald Trump’s former campaign chairman, who was prosecuted by Mueller’s office. Manafort’s gambit also failed, and he was convicted after a trial on bank and tax fraud charges in the Eastern District of Virginia and then pleaded guilty to separate conspiracy charges in the District of Columbia.

The appeals court ruling in Miller’s subpoena case comes amid the latest round of rumors that Mueller may be close to delivering some kind of report to the new attorney general, William P. Barr, which could indicate that the special counsel’s work is winding down and that he is preparing to hand off remaining matters to regular Justice Department prosecutors.

It also comes after the indictment of Stone last month on charges of lying to investigators, obstruction and witness tampering — raising the question of whether prosecutors still need Miller’s testimony, either for any effort to bring additional charges against Stone in a superseding indictment or for his eventual trial.

The appeals court held oral arguments over Miller’s appeal in November — shortly after Trump ousted then-Attorney General Jeff Sessions and installed a loyalist, Matthew G. Whitaker, as the acting attorney general, temporarily putting Whitaker in charge of Mueller. In the interim, Whitaker has since been replaced with the confirmation of Barr.

Kamenar noted the length of the delay between arguments and the ruling in Miller’s appeal, contrasting it with a separate dispute over another subpoena issued by Mueller’s office to an unidentified foreign corporation that is owned by a foreign government. In that other dispute, an appeals court panel rejected the challenge far more swiftly.