FILE PHOTO - White House Counsel Don McGahn listens during U.S. Supreme Court nominee Judge Brett Kavanaugh's U.S. Senate Judiciary Committee confirmation hearing on Capitol Hill in Washington, U.S., September 4, 2018. REUTERS/Joshua Roberts

WASHINGTON (Reuters) - The U.S. Justice Department offered President Donald Trump legal cover on Monday to block former White House counsel Don McGahn from testifying before the House Judiciary Committee, saying McGahn has “immunity from testifying” before Congress about matters related to his official duties.

The legal opinion by the Justice Department was released one day before McGahn had been due to appear before the Democratically controlled House panel by order of a subpoena to discuss matters outlined in Special Counsel Robert Mueller’s Russia investigative report.

“Congress may not constitutionally compel the president’s senior advisers to testify about their official duties,” the opinion said, citing the Constitution’s separation of powers provisions.