Washington(CNN) Retired Adm. William "Bill" McRaven resigned from the Pentagon's Defense Innovation Board last month, just days after issuing a stunning rebuke of President Donald Trump's decision to revoke the security clearance of former CIA Director John Brennan, the Pentagon confirmed Thursday.

"I can confirm that Admiral (ret) William H. McRaven resigned from the Defense Innovation Board, effective August 20, 2018," Pentagon spokesperson Heather Babb told CNN in a statement. "The Department appreciates his service and contribution on the board."

The Defense Innovation Board serves as an independent advisory committee to the Pentagon specializing in issues related to technology and innovation. McRaven's picture has been removed from the board's website.

While his departure was first reported by Defense News on Thursday, the Pentagon confirmed that McRaven, the former head of Special Operations Command, officially resigned last month, just four days after he authored a blistering op-ed in The Washington Post slamming Trump's decision to revoke Brennan's clearance.

"Through your actions, you have embarrassed us in the eyes of our children, humiliated us on the world stage and, worst of all, divided us as a nation," McRaven said of Trump's unprecedented use of a presidential authority over the classification system to strike back at one of his prominent critics.

"If you think for a moment that your McCarthy-era tactics will suppress the voices of criticism, you are sadly mistaken. The criticism will continue until you become the leader we prayed you would be," he added, also volunteering to have his own clearance revoked in an act of solidarity.

McRaven also defended Brennan as "one of the finest public servants I have ever known."

"Few Americans have done more to protect this country than John. He is a man of unparalleled integrity, whose honesty and character have never been in question, except by those who don't know him," McRaven wrote.

"Therefore, I would consider it an honor if you would revoke my security clearance as well, so I can add my name to the list of men and women who have spoken up against your presidency," he added.

McRaven, who resigned as chancellor of the University of Texas at Austin earlier this year, is widely respected among the tens of thousands of active and retired special operators.