Washington CNN  — 

Roger Stone associate Andrew Miller will have to testify to a grand jury in special counsel Robert Mueller’s investigation or will go to jail, a three-judge panel at the DC Circuit Court of Appeals ruled Tuesday morning, in an endorsement of Mueller’s authority as a prosecutor.

The court agreed with a trial-level judge’s ruling that Miller should be held in contempt of court and jailed for refusing to testify under a grand jury subpoena from Mueller.

The appeals court said that Mueller was legally appointed by Deputy Attorney General Rod Rosenstein as special counsel in May 2017. Mueller has conducted a grand jury investigation that is within his legal authority, the court wrote in its opinion.

Miller’s challenge had tested whether Mueller could call witnesses to a grand jury and take other prosecutorial steps. At least one other opponent in Mueller’s investigation, the Russian company Concord Management and Consulting, had closely watched Miller’s challenge, as a possible relief from its own criminal charge, which the company continues to fight.

The DC Circuit judges said that Rosenstein, who became acting attorney general following Attorney General Jeff Sessions’ recusal from the Russia probe, had the ability to appoint Mueller under the Constitution.

“Special Counsel Mueller effectively serves at the pleasure of an Executive Branch officer who was appointed with the advice and consent of the Senate,” the DC Circuit wrote in its 16-page opinion.

Circuit Judge Judith Rogers wrote the opinion on behalf of the panel, which also included judges Sri Srinivasan and Judge Karen Henderson. They were in complete agreement.

“The DC Circuit’s unanimous ruling is not exactly surprising, since every other federal judge to consider these challenges to Mueller’s appointment and investigation has rejected them,” said CNN Supreme Court analyst and University of Texas Law School professor Stephen Vladeck. “It’s nevertheless important because it’s the first such decision by a court of appeals – and an especially significant appellate court, at that.”

Miller has an opportunity to decide his next move, and whether he will continue to appeal.

“We are disappointed with the decision and will be considering future legal action, whether before the full court of appeals or the Supreme Court,” Paul Kamenar, Miller’s attorney, said after reading the opinion Tuesday morning.

Last summer, Miller turned over documents to Mueller’s investigators. He had worked with Stone over the years, including in 2016, when he accompanied Stone to the Republican National Convention, and received money from Stone’s super PAC for consulting work.

Kamenar heard from Mueller’s team that Miller’s testimony was still needed following Stone’s arrest for witness tampering, obstruction and lying to Congress last month. That indicated that Mueller’s grand jury investigation into Stone may still be active and working on additional indictments. The grand jury has not been spotted meeting since the day it approved Stone’s indictment.

Previously, four lower court federal judges, including one appointed by President Donald Trump, had signed off on Mueller’s work after the special counsel faced court challenges. Miller’s case is the second known time a witness in Mueller’s grand jury investigation has taken a subpoena challenge to the appeals court. In another appeal, an unnamed company owned by a foreign country lost a challenge to a summer 2018 subpoena for information. The company is now asking the US Supreme Court for review.

In late 2017, two witnesses – a lawyer and a real estate agent – were told they must testify in Paul Manafort’s case or face contempt proceedings. Both provided information to the special counsel’s office.