Roger Stone arrives for a status hearing in April at the federal court in Washington. (Joshua Roberts/Reuters)

A federal appeals court Tuesday refused to block a grand jury subpoena for testimony by Roger Stone associate Andrew Miller in an investigation launched by special counsel Robert S. Mueller III, clearing the way for a final appeal to the Supreme Court in the long-running legal dispute.

Miller was subpoenaed in June 2018 in Mueller’s probe for information about longtime Trump friend and GOP operative Stone, as well as key figures in the 2016 hacking and public release of Democratic Party emails, including by the anti-secrecy group WikiLeaks, led by Julian Assange.

Miller was also ordered to answer questions before a grand jury about Guccifer 2.0 and DCLeaks, entities that U.S. prosecutors have alleged were online fronts invented by Russian intelligence operatives to spread the hacked documents before the 2016 presidential election.

On Tuesday, Judges Karen LeCraft Henderson, Judith W. Rogers and Sri Srinivasan of the U.S. Court of Appeals for the D.C. Circuit declined to stay the subpoena after earlier refusing to rehear a February decision upholding the constitutionality of Mueller’s appointment over Miller’s challenge. The full appeals court also had declined to revisit the case.

The three-judge panel gave Miller seven days to persuade the Supreme Court to take the case.

If he fails, or the high court rules against him, Miller faces an August contempt finding by U.S. District Chief Judge Beryl A. Howell of Washington for refusing to testify, an order that would leave him facing jail if he continues to decline to appear.

Miller’s lawyer, Paul Kamenar, called the ruling “disappointing, but not unexpected.” He said he will decide shortly whether to ask Chief Justice John G. Roberts Jr. to intervene.

Mueller’s Russia investigation is over, but the special counsel’s office turned over Miller’s case to the U.S. attorney’s office for the District, which has said his testimony is still needed.

Stone has pleaded not guilty and faces trial in November on Mueller charges of lying to Congress and obstructing justice about his efforts to learn about the hacked emails.

Kamenar questioned why prosecutors still need testimony from Miller given Stone’s indictment and the release of Mueller’s final report.

“Why do they still need to hear from Andrew Miller?” Kamenar asked.

Prosecutors’ stance suggests that Miller’s testimony may be sought in one of more than a dozen pending matters that Mueller reported referring to U.S. attorney offices, or perhaps in connection with a matter for which Stone was not charged.

The parties face a looming deadline. The federal grand jury in Washington that worked with Mueller is set to end its two year term on July 5, after which unanswered subpoenas will expire.