The top career U.S. diplomat announced Thursday he will step down, dealing a blow to the State Department as the Trump administration confronts numerous international challenges. The State Department's third-ranking official, Undersecretary of State for Political Affairs Tom Shannon, informed agency staffers that he will retire as soon as a successor for his Senate-confirmed post is chosen and ready to assume the job.

Shannon is a near 35-year veteran of the U.S. Foreign Service and was the most senior department official to remain in his job after the transition from the Obama to the Trump administrations. He served under six presidents of both political parties since 1984 and was widely seen as a symbol of diplomatic continuity and professionalism during the tumultuous transition last year.

In an interview, Shannon, who holds the rank of "career ambassador" — the highest in the foreign service — said he was retiring for personal and not political reasons. "I decided it was an important moment to take a step back and to determine what next I can do in my life," Shannon said. He said the death of his mother late last year and his own 60th birthday last week contributed to his decision. But he also said he considered himself the "designated survivor" when former President Barack Obama tapped him for the undersecretary post in February 2016.

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"Aside from helping the Obama administration to the finish line, I knew my job was also about helping this institution navigate the political transition, get across the river," he said. "And, once across that river, help this institution — the foreign service and civil service — be responsive to our elected leadership."

"I've served well across 35 years," Shannon continued. "I've fought the good fight. I've lived by my oath and I want to go out living by my oath, which is respect for the Constitution, respecting our political institutions, respecting our values and respecting the choices that the American people have made."

State Spokesperson Heather Nauert issued a tweet following Shannon's announcement, saying Shannon had given State a "lifetime of service to the American people." She added, "He's a patriot, a diplomat and a great American. Working with him has been one of the great honors of my life."

Undersecretary Shannon has given a lifetime of service to the American people @StateDept. He’s a patriot, a diplomat and a great American.

Working with him has been one of the great honors of my life. He will stay on until a successor is named to ensure a smooth transition. — Heather Nauert (@statedeptspox) February 1, 2018

Secretary of State Rex Tillerson, who has been criticized for his leadership of the State Department and removing or forcing out senior career officials, said he had asked Shannon to stay on and acknowledged that Shannon's departure would be a loss. "Thirty-five years of experience is not something you replace overnight," he said.

Tillerson dropped by during the interview and said Shannon could return if he did not enjoy retirement.

Tillerson's praise aside, Shannon's departure is sure to be seized on by critics of the administration who accuse Mr. Trump and Tillerson of gutting the foreign service, particularly its senior ranks. Of five "career ambassadors" on the job when Tillerson arrived at the State Department, only one remains and that diplomat is currently on sabbatical.

The departure announcement also comes on the one year anniversary of Tillerson's tenure at the department.

Shannon, who has extensive diplomatic experience in Africa and Latin America and led U.S. delegations to Russia last year, said he had confidence in younger foreign service officers to carry on despite today's "hyper-politcized" American politics.

"It would be my hope that, independent of the political forces that swirl in this town, that especially my foreign service colleagues, both present and past, will recognize the importance of honoring the profession and our ethos of service."

Shannon said he had "supreme confidence" in the younger generation of foreign service officers, 60 percent of whom have been diplomats for less than 10 years.

CBS News' Kylie Atwood reports Under Secretary Steve Goldstein denied that Shannon's departure had anything to do with low morale at the department.

"I was with him yesterday and you could see how bittersweet he felt about this. Tillerson came in to talk to him and reinforced what a primary role he has played in his transition," said Goldstein.

Tillerson always called Shannon a "walking encyclopedia" explained Goldstein. His wealth of knowledge helped in developing strategies moving forward. Last year he was Acting Secretary of State for a few weeks during the transition. During his career Shannon served 6 presidents and 10 secretaries of state. He was the US Ambassador to Brazil, the Assistant Secretary of State for Western Hemisphere Affairs and served all over the world in different capacities.

"After more than 34 years of service to our great Republic, I have decided that it is time to step aside," Shannon wrote in a note to State Department colleagues. "My decision is personal, and driven by a desire to attend to my family, take stock of my life, and set a new direction for my remaining years."

Tillerson told Mr. Trump's chief of staff Kelly about the news yesterday and highlighted how important Shannon has been to the department. Kelly said he would like to come to Shannon's going away party.

"He told me he wanted his departure to be a model for how foreign service officers should leave their post. There is no doubt he is beloved in the department," said Goldstein.