Washington (CNN) Andrew Miller, a former associate of longtime Donald Trump confidant Roger Stone, did not appear for a scheduled grand jury hearing on Friday, defying a subpoena from special counsel Robert Mueller, according to his attorney.

Miller's attorney Paul Kamenar told CNN that they intend to appeal the order to appear before the grand jury, and he said the first step would be for him not to appear.

In a closed hearing Friday in a Washington, DC, district court, Judge Beryl Howell found Miller in contempt and then stayed that ruling until this Monday to allow for an appeal to be filed.

"Our next stop is the court of appeals," Kamenar told CNN on Friday. Asked how long he thinks the process will take, he said, "This may be expedited," but added, "it eventually would be headed to the Supreme Court for the final word on this."

Miller was ordered to appear before the Mueller grand jury last week after a federal judge rejected his attempt to quash the subpoena on the grounds that Mueller's appointment was unconstitutional. The judge wrote in a 93-page opinion that Mueller's appointment was valid.

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