One of former special counsel Robert Mueller's longtime aides will appear alongside him during his highly-anticipated testimony before the House Judiciary Committee, a spokesperson said Tuesday, but is not expected to be sworn in.

Mueller's team made a last-minute request that Aaron Zebley be sworn in and testify with him during his scheduled hearings before Congress on Wednesday, a congressional source familiar with the request told NBC News.

Mueller is slated to testify on his report into Russian interference in the 2016 election and the country's influence on President Donald Trump for three hours before the House Judiciary Committee, take a break, then appear for at least two additional hours before the House Intelligence Committee.

For the first hearing, Zebley will sit alongside Mueller as his counsel, according to the Judiciary Committee spokesperson. The committee, however, is not updating its guidance to include Zebley as a witness. This means that Zebley will not be sworn in. Mueller can confer with him as he is questioned by the panel, according to committee rules, but cannot answer questions.

The ranking Republican on the Judiciary Committee, Rep. Doug Collins of Georgia, said Tuesday that GOP members had "not gotten assurances from the House Democrats on the committee that he [Zebley] will not speak."

"He's not supposed to speak in that role to anyone on the committee or asked questions. And we're asking, and, frankly, that that be confirmed before the hearing. So we don't have to waste time with it tomorrow," Collins said.

Jim Popkin, Mueller's spokesperson, disputed the idea that Zebley's presence at the hearings amounted to an 11th-hour addition.

“Aaron Zebley was the Deputy Special Counsel and had day-to-day oversight of the investigations conducted by the Office," Popkin said in a statement Tuesday. "He will accompany Special Counsel Mueller to the Wednesday hearings, as was discussed with the committees more than a week ago."

Rep. Adam Schiff, D-Calif., the chairman of the House Intelligence Committee, said Tuesday during a conversation at the Center for American Progress that his panel was still working out what Zebley's function might be.

However, an aide for the House Intelligence Committee told NBC News Tuesday evening that Zebley will be sworn in for the hearing.

Trump lashed out about Zebley's role in Mueller's testimony, tweeting Tuesday night that the decision "very unfair."

"Just got back only to hear of a last minute change allowing a Never Trumper attorney to help Robert Mueller with his testimony before Congress tomorrow," Trump said. "What a disgrace to our system. Never heard of this before."

Just got back only to hear of a last minute change allowing a Never Trumper attorney to help Robert Mueller with his testimony before Congress tomorrow. What a disgrace to our system. Never heard of this before. VERY UNFAIR, SHOULD NOT BE ALLOWED. A rigged Witch Hunt! — Donald J. Trump (@realDonaldTrump) July 23, 2019

Zebley is especially close to Mueller, serving as his chief of staff at the FBI when Mueller was the director. Zebley is also an alumnus of the law firm WilmerHale, where Mueller worked after leaving the FBI.

Zebley followed Mueller to the Justice Department when he was tapped to be special counsel in 2017. Zebley is also a former FBI agent who was involved in an international hunt for al Qaeda terrorists before the Sept. 11, 2001, terrorist attacks.

In June, Mueller agreed to testify before Congress about his Russia investigation after he was subpoenaed by Rep. Jerry Nadler, D-N.Y., the chairman of the House Judiciary, and Schiff.

Popkin said Monday that the former special counsel will make a brief opening statement when he testifies about his 22-month investigation, and will offer his 448-page report as his full statement for the record.

In May, Mueller stressed at a press conference that if he were called to testify he would stick closely to his written report. "Any testimony from this office would not go beyond our report. It contains our findings and analysis, and the reasons for the decisions we made," he said.

"We chose those words carefully, and the work speaks for itself. The report is my testimony. I would not provide information beyond that which is already public in any appearance before Congress.”

In a letter on Monday, the Justice Department also issued stern guidance to Mueller, stating that "any testimony must remain within the boundaries of your public report because matters within the scope of your investigation were covered by executive privilege."

CORRECTION (July 25, 2019, 10:31 p.m. ET): A previous version of this article misstated the first name of Robert Mueller's longtime aide in a headline. He is Aaron Zebley, not Adam.