British top official Sir Mark Sedwill said on Sunday he would step down as it was announced that part of his work would go to the UK's leading Brexit negotiator.
His exit could pave the way for a shift in government leadership at the highest level, with Prime Minister Boris Johnson's oldest and most controversial advisor Dominic Cummings expecting to oversee such reforms.
In a statement, Sedwill said he would leave his post in September after serving under seven prime ministers in various jobs.
Sedwill's job was not just to be head of the civil service, but also to be Secretary of the Cabinet and National Security Advisor.
The national security post is now being taken over by Brexit negotiator David Frost.
"I look forward to contributing to the Prime Minister's vision for a global Britain with real influence around the world," said Frost.
He will remain the negotiator for the rest of the Brexit talks.
The UK is still working on its final agreement with the European Union as the deadline approaches at the end of the year. Some believe that it could eventually leave without an agreement.
Johnson said he was confident that Frost would "improve this country's ability to project influence for the better."
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