WASHINGTON - The spokesman for President Donald Trump's National Security Council plans to leave the administration, the White House said on Sunday, a day before Trump's third national security adviser formally takes up his post.Michael Anton, who also worked for former President George W. Bush's National Security Council and is a former BlackRock managing director, is leaving after serving under Trump for more than a year, White House press secretary Sarah Sanders said in a statement."Michael is one of the smartest and most talented individuals I’ve ever worked with," Sanders said. "He will be greatly missed."Trump telephoned Anton on Sunday to thank him for his service, a White House official said.The White House did not offer a reason for Anton's departure or say exactly when he would leave. The departure is the latest in a string of staff changes in the administration.Anton told Politico he planned to join Hillsdale College’s Kirby Center in Washington as a writer and lecturer. Anton did not immediately respond to a Reuters request for comment.Anton published a widely read essay in conservative circles before the 2016 election under a pseudonym that compared the presidential race to the doomed Sept. 11, 2001, flight that crashed in Pennsylvania after passengers charged the cockpit."2016 is the Flight 93 election: charge the cockpit or you die," he wrote. "To compound the metaphor: a Hillary Clinton presidency is Russian Roulette with a semi-auto. With Trump, at least you can spin the cylinder and take your chances."Last month, Trump said he was replacing H.R. McMaster as national security adviser with John Bolton, a hawk who has advocated using military force against North Korea and Iran. A week earlier, Trump fired Rex Tillerson as secretary of state and nominated Central Intelligence Agency Director Mike Pompeo to replace him.Bolton begins work on Monday.McMaster replaced Michael Flynn, who resigned soon after Trump took office following disclosures he had discussed U.S. sanctions on Russia with the Russian ambassador to the United States before Trump took office.Flynn pleaded guilty in December to lying to the FBI about his contacts with Russia.