A congressional coalition of former military pilots sent a letter to President Biden demanding Secretary of Defense Lloyd Austin’s resignation or immediate dismissal amid ongoing questions surrounding his medical treatment in the ICU without the White House’s knowledge for days. 

"We believe Secretary Austin’s blatant violation of the Pentagon’s Principles of Information and serious lapse in judgment warrants his immediate resignation, as well as the resignation of any staff involved in covering up his hospitalization. If he does not resign, he should be immediately dismissed," the MACH 1 Caucus wrote to Biden in a letter obtained by Fox News Digital on Tuesday. 

The MACH 1 Caucus comprises Reps. August Pfluger, R-Texas, Mike Garcia, R-Calif., Jake Ellzey, R-Texas, and Scott Franklin, R-Fla., all former military pilots. The caucus was officially announced in February 2022 and advocates for fighter pilots and "educating other members" in Congress on military pilot-related issues, the caucus said when it was officially formed. 

Pfluger, who spearheaded the letter, told Fox News Digital that Austin’s absence in a hospital room without the president’s knowledge for days is "a complete dereliction of duty."

"As a former combat commander, I know how important it is to have secondary layers of authority in place during times of absence." Pfluger said. "These contingency plans have life-or-death implications. Secretary Austin has weakened our military from day one – by advocating for a woke and weak military and now by abdicating his post without properly appointing a proxy or notifying the President of the United States or the National Security Council. 

"This is an incredible threat to our national security and a complete dereliction of duty. Secretary Austin, and any staff members involved in covering up his hospitalization, must resign or be dismissed."

Calls for Austin to resign or be fired have rung out repeatedly among conservative leaders and lawmakers since last week, when the media was made aware on Friday of Austin’s hospital treatment. Reports quickly followed that Biden, the National Security Council and top Pentagon leaders – including Austin’s deputy, Kathleen Hicks – were kept in the dark regarding the hospitalization for days. The Pentagon's press secretary told the media Monday that it took so long to flag the White House and National Security Council of the hospitalization because Austin's chief of staff had the flu. 

On Tuesday, it was revealed Austin underwent a prostatectomy late last month after a prostate diagnosis and was admitted to the ICU last week due to developing a urinary tract infection following the procedure. 

"It is unacceptable and unconscionable that the principal cabinet member responsible for U.S. national security would be absent without notice of leave, designating an alternative chain of command, or making relevant Members of Congress and the Executive Branch aware of such an absence. At a time of significant global instability as Ukraine defends itself against Russia and a regional conflict continues in the Middle East, it is dangerous, reckless, and beyond concerning that the individual you have designated to lead our men and women in uniform does not follow the proper protocol when undergoing surgery," the MACH 1 Caucus wrote in their letter to Biden. 

"This failure to notify the proper officials is an extraordinary breach of protocol, and at most, dereliction of duty," the caucus added. 

Fox News Digital spoke to former assistant U.S. attorney and Fox News contributor Andy McCarthy earlier this week, who detailed that there is a clear chain of command within the Department of Defense and federal government at large, and that Austin "undermined" the U.S. "constitutional commitment to civilian control of the military" and the nation's "readiness in a time of crisis" when his admission to the ICU was concealed from the White House.

"The Constitution reposes all executive power in a single official, the president, and makes the president commander in chief of the armed forces. This ensures civilian control of the armed forces: the U.S. military does not have independent constitutional standing – it answers to the chief executive and relies for its existence, operations and armaments on congressional legislation," McCarthy said. 

"Austin’s behavior was reckless, particularly at a time when war has broken out in Europe and the Middle East, China is a major threat in the Pacific, global commerce is being paralyzed by piracy in the Red Sea, and American forces are not only in harm’s way but being attacked by Iranian proxies. By keeping his incapacity a secret from the White House, he undermined our constitutional commitment to civilian control of the military and undermined U.S. readiness in a time of crisis," McCarthy added in his comment earlier this week. 

The MACH 1 Caucus is calling on Biden to answer a series of questions by Jan. 23, such as whether "any key national security decisions [were] made during [Austin’s] absence;" details and a timeline on communication between Austin and federal government leaders that ensured "our military's chain of command was intact should there have been an emergency."

"What specific disciplinary action, if any, will Secretary Austin and any staff members who failed to meet the standards of public disclosure for senior government officials unable to exercise their duties?" they asked. 

The Pentagon and White House have pushed back on calls for Austin to resign or be fired, with a White House official telling Fox News Digital earlier this week that Biden "continues to have full trust and confidence in the Secretary."

The Pentagon said Monday evening that the Department of Defense will review the process by which top leaders are notified of the transfer of duties from the secretary of defense. 

"The department recognizes the understandable concerns expressed by the public, Congress and the news media in terms of notification timelines and DOD transparency. Now, I want to underscore again that Secretary Austin has taken responsibility for the issues with transparency, and the department is taking immediate steps to improve our notification procedures," Pentagon press secretary Maj. Gen. Pat Ryder said Tuesday. 

The caucus called on Biden to schedule a meeting with the lawmakers to further discuss Austin’s hospitalization and the "breach of protocol."

The White House on Wednesday morning did not return a request for comment on the MACH 1 letter.