By Ian Dunt Follow @IanDunt
The power structure in Whitehall was dramatically unsettled today with the stepping down of Sir Gus O'Donnell and a major shake-up of roles.
His dual role as Cabinet secretary and head of the civil service will be separated, with current permanent secretary at No 10 Jeremy Heywood taking on the former role.
"I am passionate about the importance of our traditional values of honesty, objectivity, integrity and impartiality," Sir Gus said.
"I am also confident that the civil service will demonstrate the professionalism and pace to face the challenges of delivering better services with fewer resources. I wish Jeremy every success in his new role."
Sir Gus will leave at the end of the year. David Cameron has already nominated him for a life peerage.
"Sir Gus O'Donnell has been the outstanding civil servant of his generation," the prime minister said.
"His support during the formation of the coalition government, and in ensuring the smooth and effective running of Cabinet government since I took office, has been invaluable.
"He has given dedicated and professional service under five prime ministers, the last four of whom he has worked with very closely."
Sir John Major took Sir Gus with him to Downing Street as his press secretary after working with him as chancellor. He then returned to the Treasury before going back to Downing Street as Cabinet secretary in 2005.
Sir Gus had a famously difficult relationship with Gordon Brown, who was prone to fearsome rages and some believed him to appear visibly relived when Mr Cameron entered Downing Street.
Ed Miliband commented: "Gus O'Donnell has had a distinguished career serving our country, including at the Treasury and the Cabinet Office.
"I had a chance to work with him in both of those places, and he was an outstanding public servant, unfailingly helpful, thoughtful and supportive in implementing the agenda of the government."
The new Cabinet secretary will continue to provide senior policy advice to the prime minister and deputy prime minister and act as secretary to the Cabinet.
The head of the civil service will provide professional and corporate leadership to the civil service.
Both roles will report directly to the prime minister.
Sir David Normington, first civil service commissioner, will launch an internal Whitehall competition to recruit a head of the civil service.