The White House has directed former White House counsel Don McGahn not to appear before Congress, backed by a Justice Department legal opinion released Monday claiming Congress cannot compel the former White House lawyer to testify. The House Judiciary Committee has been informed that McGahn will not appear Tuesday, when he had been subpoenaed and scheduled to testify.

The Office of Legal Counsel opinion invoked the separation of powers to argue that McGahn should not appear before the congressional panel.

"The immunity of the president's immediate advisers from compelled congressional testimony on matters related to their official responsibilities has long been recognized and arises from the fundamental workings of the separation of powers," the opinion reads. "Those principles apply to the former White House Counsel. Accordingly, Mr. McGahn is not legally required to appear and testify about matters related to his official duties as counsel to the president."

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In a letter to House Judiciary Committee Chairman Jerry Nadler, the current White House counsel, Pat Cipollone wrote, "There is no question that the position of counsel to the president falls within the scope of the immunity."

Nadler has threatened to hold McGahn in contempt if he does not appear. McGahn's attorney, William Burck, wrote to Nadler to disagree with the position taken by the committee, but said that McGahn is bound to "honor his ethical and legal obligations as a former senior lawyer and senior advisor to the President."

McGahn is a key figure in special counsel Robert Mueller's report. Not only is he cited more than any other witness in the report, it's McGahn who told Mueller's team that the president on multiple occasions had directed him to fire Mueller, an order McGahn refused.

White House press secretary Sarah Sanders released a statement saying that the move to keep McGahn from testifying "has been taken to ensure that future Presidents can effectively execute the responsibilities of the office of the presidency."

That reasoning was echoed by the president as he left for a rally in Pennsylvania Monday afternoon. Asked by reporters about the McGahn opinion, he said, "As I understand it [Justice Department lawyers] are doing it for the office of the presidency -- for future presidents. I think it's a very important precedent. And the attorneys say they're not doing that for me — they're doing that for the office of the president. So, we're talking about the future."

Mr. Trump had already suggested he didn't think McGahn should testify.

"I've had him testifying already for 30 hours and it's really — so I don't think I can let him and then tell everybody else you can't," Trump said Thursday. "Especially him, because he was a counsel, so they've testified for many hours, all of them, many, many, many people. I can't say, 'Well, one can and the others can't.' I would say it's done."

CBS News White House correspondents Paula Reid and Weijia Jiang and CBS News' Kathryn Watson contributed to this report.