Britain's most senior civil servant stepped down on Sunday after reports of clashes with Prime Minister Boris Johnson's top political adviser, part of what is expected to be a wider shake up of officials at the heart of power.

Mark Sedwill, a career diplomat, was appointed cabinet secretary and national security adviser by Johnson's predecessor Theresa May. 

In a letter to Johnson released on Sunday, he wrote that having stayed on for "the acute phase" of the coronavirus crisis he was now leaving as "the government's focus is now shifting to domestic and global recovery and renewal." He said he would leave government service at the end of September.

Johnson's senior political adviser, Dominic Cummings, has long been critical of the civil service, saying government is not nimble enough to deliver change. Some newspapers reported he warned aides last week that a "hard rain is going to fall."

Earlier this year, Johnson forced the resignation of his finance minister, tightening his control over the treasury, and earlier this month he announced the merger of the government's diplomatic and aid departments.

Johnson's office released a letter in which he thanked Sedwill for his "outstanding service," and asked him to lead a new G7 panel on global economic security as Britain assumes the presidency of the rich nations' group and hosts its summit next year.

David Frost, Britain's chief negotiator with the European Union, will replace Sedwill as national security adviser at around the end of August and there will be a competition to appoint a new cabinet secretary and head of the civil service.

"I look forward to helping deliver the prime minister's vision for a global Britain, with real influence around the world," Frost said. 

He will remain as chief negotiator for the remainder of the Brexit talks. Britain is still hammering out its final deal with the European Union as the end-of-year deadline approaches, with some believing it could eventually leave without any agreement being reached. 

Johnson said he was confident Frost would make a "difference to this country's ability to project influence for the better."

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Questions over timing

Britain's main opposition Labour Party raised questions over the timing of the move. Helen Hayes, policy chief shadowing the cabinet office, said: "On the day it was revealed millions of jobs across the country could be under threat in the coming months, it is very concerning that Boris Johnson and Dominic Cummings are preoccupied with reshuffling Whitehall."

Johnson's team has been criticized over its response to the coronavirus crisis, with opposition parties and some scientists arguing it was too slow to tackle the pandemic.

Britain now has one of the world's highest death tolls from the disease and both the Bank of England and government budget forecasters have warned of the potential for unemployment to surge as the government phases out temporary support measures for workers hit by lockdown measures.

Sedwill also bore the brunt of criticism from Brexit supporters during Britain's earlier negotiations with the EU over its exit from the bloc, with some Conservatives accusing him of not doing enough to prepare for a so-called no deal departure.

Earlier on Sunday, Home Secretary Priti Patel hinted that there could be changes.

"This is the people's government, delivering on the people's priorities, and effectively any reforming government will be based around the type of delivery that our prime minister wants to drive for our great country and obviously have the right kind of support around him to deliver that," she told Sky News.

(With input from Reuters, AFP)