FILE - In this file photo dated Tuesday April 28, 2020, Britain's Prime Minister Boris Johnson, centre, Chancellor of the Exchequer Rishi Sunak, right, and Cabinet Secretary Mark Sedwill stand inside 10 Downing Street, London, during a minute's silence to pay tribute to the NHS staff and key workers who have died during the coronavirus outbreak. Britain’s top civil servant Mark Sedwill announced Sunday June 28, 2020, he will resign from his role as Cabinet Secretary, national security adviser and head of the Civil Service in September 2020. (Stefan Rousseau/Pool via AP)
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LONDON (AP) — Britain’s top civil servant and one of the most powerful people in government announced Sunday that he would step down, as Prime Minister Boris Johnson prepared to shake up the the country’s civil service.

Mark Sedwill said Sunday he would resign from his roles as Cabinet secretary, national security adviser and head of the Civil Service in September. He said it had been a privilege to serve, but now is the time for change because the Johnson government was shifting to a new phase.

"It was obviously right to stay on for the acute phase of the COVID-19 crisis," Sedwill said in his resignation letter. “As you are setting out this week, the government’s focus is now shifting to domestic and global recovery and renewal.”

Johnson’s Europe adviser, David Frost, will step into the role of national security adviser. The statement from Johnson's Downing Street office didn't say who was stepping into the other roles.

Frost will remain the chief negotiator for European Union talks and said “these will remain my top single priority until those negotiations have concluded, one way or another.''

Sedwill, a 54-year-old former diplomat, has been accused of lacking the domestic policy skills needed to respond to the crisis sparked by the coronavirus pandemic. In recent months, reports also had suggested he had fallen afoul of Dominic Cummings, Johnson's powerful aide, who wanted the public servant removed.

Speculation about his future was fueled when Simon Case, who was appointed permanent secretary in Johnson's Downing Street office amid the pandemic, was chosen to lead a review into social distancing restrictions. The review led to an easing of lockdown measures in place since March 23.

Case is considered the favorite to become the next Cabinet secretary.

Dubbed the ultimate "securocrat," Sedwill was a trusted lieutenant of Johnson’s predecessor, Theresa May, during her time as prime minister.

Johnson gave Sedwill a new task, asking him to lead a new Group of Seven nations panel on Global Economic Security as the U.K. assumes the presidency with a mission to ensure that “the global free trading system on which our economy is based remains fair, competitive and secure.”

Johnson praised the outgoing civil servant for 30 years of "outstanding," work.

"It has been by any standards a massive contribution — but as PM I have particularly appreciated your calm and shrewd advice,” he wrote to Sedwill as he accepted his resignation.