LONDON: Britain's top civil servant Sir Mark Sedwill said Sunday he will stand down as it was revealed part of his job will go to Britain's senior Brexit negotiator.

His exit could pave the way for a shake-up of how the civil service is run at the very highest level, with Prime Minister Boris Johnson's most senior - and controversial - adviser Dominic Cummings expected to oversee any such reforms.

In a statement, Sedwill said he would leave his post in September after serving in various jobs under seven prime ministers.

Sedwill's job encompassed not only being head of the civil service, but also cabinet secretary and national security adviser.

The national security post will now be taken over by Brexit negotiator David Frost.

"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".