Former White House Counsel Don McGahn must testify about his time working for President Donald Trump, a federal judge ruled on Monday.

U.S. District Judge Ketanji Brown Jackson of the District of Columbia said McGahn must comply with a subpoena for his testimony issued by the House Judiciary Committee prior to the formal launch of congressional Democrats’ impeachment inquiry. However, Brown Jackson stated the former White House counsel could “invoke executive privilege where appropriate” during his hearing before lawmakers. The Trump administration is likely to appeal the decision.

“It is clear to this Court for the reasons explained above that, with respect to senior-level presidential aides, absolute immunity from compelled congressional process simply does not exist,” said the judge.

The White House has argued that McGahn and other witnesses have “absolute immunity” from testifying.

But Jackson disputed the administration’s reasoning in a 118-page ruling. “That is to say, however busy or essential a presidential aide might be, and whatever their proximity to sensitive domestic and national-security projects, the President does not have the power to excuse him or her” from complying with a valid congressional subpoena, Jackson wrote. She is an appointee of President Barack Obama.

Whether McGahn has to provide all the information Congress seeks, though, is another matter, the judge wrote. The president may be able to assert “executive privilege” on some sensitive issues, she wrote.

McGahn “will comply with Judge Jackson’s decision unless it is stayed pending appeal. The DOJ is handling this case, so you will need to ask them whether they intend to seek a stay,” a lawyer for the former White House aide said in a statement.

Update—6:28 P.M. EST: The DOJ announced it will appeal the ruling and seek a stay to block enforcement of it.

The Associated Press contributed to this report.