White House Homeland Security Advisor Thomas Bossert resigned Tuesday, just one day after President Trump’s new national security adviser, Ambassador John Bolton, began his tenure in the National Security Council.

White House officials confirmed Bossert’s resignation to Fox News Tuesday morning.

“The President is grateful for Tom’s commitment to the safety and security of our great country,” White House Press Secretary Sarah Sanders said in a statement. “Tom led the White House’s efforts to protect the homeland from terrorist threats, strengthen our cyber defenses, and respond to an unprecedented series of natural disasters. President Trump thanks him for his patriotic service and wishes him well.”

Bossert served as a deputy homeland security adviser to former President George W. Bush, and has held positions in the Federal Emergency Management Agency, the Small Business Administration, the Office of the Independent Counsel, and the House of Representatives. Bossert was also appointed as the director of Infrastructure Protection under Bush.

A White House source told Fox News Bossert’s resignation is part of Bolton’s “cleaning house” at the NSC.

A national security source also told Fox News that Bolton is considering a merger of the NSC and Homeland Security Council (HSC) within the White House. Right now, the NSC and HSC function as separate bodies, with their own meetings and policy issues, but share a staff.

Under the George W. Bush administration, the HSC and NSC worked as separate staffs. During the early years of the Obama administration, HSC and NSC merged, but later split.

The NSC primarily focuses on U.S. foreign policy and nuclear proliferation, while the HSC focuses on national defense-related issues like terrorism and border security.

If Bolton does merge the HSC and NSC, the bodies would hold joint meetings on mutually-agreed upon issues, and provide joint policy recommendations to the president, a national security source told Fox News.

President Trump appointed Bolton to replace National Security Adviser H.R. McMaster just last month. Bolton officially began at the White House on Monday.

Bolton has previously served in the administrations of presidents Ronald Reagan, George H.W. Bush, and George W. Bush, and served as a Bush lawyer during the 2000 Florida recount. Bolton served as U.S. permanent representative to the United Nations from 2005 to 2006, and as undersecretary of state for arms control and international security from 2001 to 2005.

The position of White House national security adviser does not require Senate confirmation.

Bolton is now Trump’s third national security adviser—replacing McMaster, who replaced Michael Flynn.

Fox News' John Roberts and Gillian Turner contributed to this report.