An IRS whistleblower who claims to have information about alleged mishandling and political interference in the ongoing criminal probe into Hunter Biden has been removed from the investigation into the president's son, attorneys for the whistleblower have told key congressional lawmakers.

Attorneys for the whistleblower sent a letter on Monday to a series of House and Senate Committee chairmen informing them of the change, adding that the whistleblower's investigative team has also been taken off the case, according to a copy of the letter obtained by CNN.

The attorneys — including Tristan Leavitt, President of Empower Oversight, and Mark Lytle — added that the whistleblower was "informed the change was at the request of the Department of Justice."

The letter states that it is an investigation of a "high-profile, controversial subject," which sources familiar with the matter have previously confirmed to CNN was Hunter Biden.

Federal prosecutors have spent years, spanning three attorneys general, investigating Hunter Biden and have weighed bringing charges against him for alleged tax crimes and a false statement, CNN has previously reported. So far, no charges have been filed. Hunter Biden has denied wrongdoing.

The IRS and an attorney for the IRS whistleblower didn't immediately respond to a request for comment. A spokesperson for the US Attorney's Office in Delaware, the office conducting the Hunter Biden criminal probe, declined to comment.

"The department cannot comment on the matter," a Justice Department spokesperson said. "As to any investigation of Hunter Biden, as the attorney general has said, that investigation is being handled by US Attorney David Weiss, who has full authority to make investigative decisions and to bring charges in any jurisdiction as he deems appropriate."

CNN has also reached out to Hunter Biden's legal team to request comment.

White House spokesman Ian Sams previously said that "President Biden has made clear that this matter would be handled independently by the Justice Department, under the leadership of a US Attorney appointed by former President Trump, free from any political interference by the White House. He has upheld that commitment."

Rep. Richard Neal, the top Democrat on the House Ways and Means Committee said in a statement, "Ways and Means Democrats take whistleblowers and their protections seriously."

"More detail is needed here, and we call on Commissioner Werfel to provide the necessary context before making any further judgment," the statement said.

Leavitt and Lytle claim in their letter that the move "is clearly retaliatory and may also constitute obstruction of a congressional inquiry" -- a claim that stands in contrast to testimony made by IRS commissioner Daniel Werfel in April, when he told the House Ways and Means Committee that, "I can say without any hesitation there will be no retaliation for anyone making an allegation or a call to a whistleblower hotline."

The attorneys write that, given the agent has successfully sought whistleblower protections, he is protected from retaliatory personnel actions and that "any attempt by any government official to prevent a federal employee from furnishing information to Congress is also a direct violation of longstanding appropriations restriction," and that it is "a crime to obstruct an investigation of Congress."

The move to sideline the whistleblower and his investigative team comes months after some agents complained that they were no longer being included in some discussions about the case, in part because of alleged leaks, people familiar with the matter told CNN.

The investigative work on the case is largely over and the case awaits a charging decision by US Attorney David Weiss, a Trump appointee who is overseeing the probe, CNN has previously reported.

The dispute is also not uncommon in fights between Congress and the executive branch over whistleblower complaints. There's no indication the agent in question has lost their job at the IRS.

The letter was sent to Democratic and Republican chairmen as well as the ranking members on the Senate Finance and Judiciary committees and the House Ways and Means and Judiciary committees. Leavitt and Lytle asked the committee leaders to give the matter their "prompt attention."

Republicans on the House Ways and Means Committee confirmed receipt of the letter to CNN, adding in a statement that the "most recent communication concerning allegations of retaliatory actions against a whistleblower appears to contradict the sworn testimony by Commissioner Werfel who pledged that whistleblower protections would be upheld."

"Congress's duty to hold government agencies accountable relies on the availability and willingness of individuals to speak out about wrongdoing. Not only does retaliation like this discourage whistleblowers, it can also rise to the level of an illegal violation of statutory protections for whistleblowers," the statement continued, going on to say that Republicans on the panel are calling on "Commissioner Werfel to abide by his pledge, quickly provide information to Congress in response to these allegations, and ensure that no action is taken to discourage those who attempt to shine a light on government misconduct."

CNN has previously reported that the IRS whistleblower alleges that Weiss' ability to bring charges in the case involving Hunter Biden is being thwarted by political appointees. That would contradict promises by Attorney General Merrick Garland in congressional testimony that Weiss, a Donald Trump appointee, has full authority to make final decisions on whether to prosecute the president's son.

The IRS whistleblower also claimed to have information that "contradicts sworn testimony to Congress by a senior political appointee," according to a previous letter from his attorney. That senior political appointee was Garland, people familiar with the matter previously told CNN.

Attorneys for the IRS whistleblower met with congressional investigators earlier this month, laying the groundwork for what their client hopes to share with Congress.

Preparations for that potential interview are underway, and the meeting is expected to take place in the near future, sources familiar with the discussion tell CNN.