John Kelly will leave his role as Donald Trump’s White House chief of staff “at the end of the year”, the president said on Saturday. The retired Marine Corps general has spent 16 months in the position.

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Trump spoke to reporters gathered on the South Lawn of the White House, as he left Washington for the army v navy football game in Philadelphia.

“John Kelly will be leaving – I don’t know if I can say ‘retiring’,” the president said. “But he’s a great guy. John Kelly will be leaving at the end of the year. We’ll be announcing who will be taking John’s place … I’ll be announcing that over the next day or two.”

Nick Ayers, the 36-year-old chief of staff to the vice-president, Mike Pence, is Kelly’s likely successor.

An unnamed White House official told the Associated Press on Saturday Ayers and Trump had discussed the job for months and were working out terms. Trump wants his next chief of staff to serve through the 2020 election. Ayers, who has young children, had planned to leave the administration at the end of this year.

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News outlets have reported dissent in the White House over the rise of Ayers, which Politico said this week could lead to staff departures.

The website quoted a “former White House official” as saying: “People are threatened by Nick’s age and his reputation as this young political savant. He has the endorsements of Jared [Kushner], Ivanka [Trump] and Pence, but not a lot of fans beyond that.”

The Wall Street Journal reported that the relationship between Kelly and Trump had completely broken down, leading the president to tell an associate to “stop calling John” and to instead “call Nick, he’s my guy”.

On Saturday, Trump said: “John will be leaving at the end of the year. He’s been with me almost two years now.”

Kelly enlisted in the Marines in 1970 and served for nearly 50 years. He did not go to Vietnam but he did see extensive action in Iraq before rising to lead US Southern Command. One of a number of generals appointed to key roles, including the defense secretary, Jim Mattis, and former national security advisers Michael Flynn and HR McMaster, he was Trump’s first homeland security secretary. In July 2017, he replaced former Republican National Committee chair Reince Priebus at the head of White House operations.

Trump reportedly wanted him to impose order on a White House riven by factionalism and rivalry. But reports of his exasperation with Trump and impending departure, whether by choice or forced, quickly became a feature of Washington gossip, not least after the veteran journalist Bob Woodward wrote in his bestselling book Fear that Kelly called Trump an “idiot” at the head of a “Crazytown” administration.

Kelly dutifully denied such reports and stayed in post. But by virtue of his proximity to the unpredictable president, he often became uncomfortably embroiled in controversy. Sometimes, it surrounded apparent public expressions of exasperation with Trump’s outbursts. At other times, Kelly’s military background was dragged into play.

In France in November for events marking the 100th anniversary of the end of the first world war, Kelly visited an American cemetery outside Paris while Trump remained in the city, the White House insisting rain made a presidential visit impractical.

Facebook Twitter Pinterest Donald Trump speaks to reporters outside the White House in Washington DC on 8 December. Photograph: Olivier Douliery/POOL/EPA

In Washington in October 2017, Kelly appeared at the White House podium to attack a congresswoman who criticised Trump’s handling of a call to the mother of a serviceman killed in Niger.

It was reported this week that Kelly has been interviewed as part of special counsel Robert Mueller’s investigation of Russian election interference, links between Trump aides and Moscow and potential obstruction of justice by the president.

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As chief of staff, Kelly carried out a number of firings decided by Trump, including that of adviser Omarosa Manigault-Newman, who surreptitiously recorded the meeting and released the tape to the press. It was also reported he called Jeff Sessions to request his resignation as attorney general after the midterm elections.

Last month, NBC News reported that Kelly frequently clashed with Melania Trump over personnel and logistical issues.

Trump made another staffing announcement on Saturday, saying in a tweet that he was nominating army Gen Mark Milley as the next chairman of the joint chiefs of staff, replacing Gen Joe Dunford, a marine. The announcement had not been expected until the new year.

On his flight to the army v navy football game, Trump also tweeted a familiar and vituperative attack on the Connecticut Democratic senator Richard Blumenthal, regarding his claims about service in Vietnam.

Blumenthal received at least five deferments before serving in the Marines Corps Reserve but did not deploy to Vietnam, which he later claimed to have done. Trump never served in the armed forces. He received five draft deferments during the Vietnam war, four on academic grounds and one because of problems with his feet.