Russian President Vladimir Putin has unexpectedly dismissed his chief of staff Sergei Ivanov.
Mr Ivanov has been part of Mr Putin's trusted inner circle for many years.
The 63-year-old has now been made a special representative for environmental and transport issues.
A statement from the Kremlin said that Mr Putin had "decreed to relieve Ivanov of his duties as head of the Russian presidential administration", but gave no reason.
Mr Ivanov's deputy since 2012, Anton Vaino, has been appointed as his successor.
Mr Vaino, 44, is a former diplomat.
Born in the Estonian capital Tallinn in 1972, he graduated from the prestigious Moscow State Institute of International Relations (MGIMO) and served in the Tokyo embassy. Later he managed presidential protocol and government staff, the Kremlin website says (in Russian).
On being appointed, he told Mr Putin: "Thank you for your trust. I think the administration's most important task is to support your activity as head of state in terms of drafting laws and control over how your instructions are implemented."
Mr Putin told a Russian TV station on Friday that Mr Ivanov had asked to leave the post, and recommended that Mr Vaino should replace him.
Do the smiles convince? By the BBC's Sarah Rainsford in Moscow
This is a move that has mystified Moscow. Sergei Ivanov has long been one of Vladimir Putin's closest allies and, like him, served in the Soviet security service, the KGB.
As chief of staff he was one of the most powerful men in the country. In a meeting with President Putin shown on state television both men claimed that the chief of staff was stepping down at his own request.
But despite the smiles for the cameras, few here are convinced - especially now, just before parliamentary elections.
So is this the fall-out from some kind of power struggle? No-one knows yet. But the official claim - that a man once touted as a potential president, suddenly wanted to run Russia's environmental policy - has been met with great scepticism.
In remarks to Mr Putin, quoted on the Kremlin website, Mr Ivanov said "it's true that in early 2012 I asked you, in a conversation, to entrust me with this very complicated post, even - you could say - troublesome post, for four years.
"Well, it turns out that I've been presidential chief of staff for four years and eight months."
Secret service ties
Mr Ivanov took up the post in December 2011. He served previously as a deputy prime minister and defence minister.
He is a member of the Russian Security Council and a former member of the KGB state security service, like Mr Putin.
In the late 1990s, when Mr Putin was head of the Federal Security Service (FSB), which replaced the KGB, Mr Ivanov was appointed as his deputy. When Mr Putin came to power, he named Mr Ivanov as one of the five people he trusted most.
It was once thought that Mr Ivanov might become president of Russia after Mr Putin's second term, as a third term for Mr Putin would have been unconstitutional.
But that post was taken by another close Putin ally, Dmitry Medvedev. Mr Putin became prime minister, before returning to the presidency just three-and-a-half years later.