The Cabinet Office says the move is "by mutual agreement" but it follows months of negative media reports about his performance.
In a statement, Sir Keir said: "I am very grateful to Sir Chris for his long and distinguished career of public service, spanning more than 35 years, and for the support that he has given me over the past year.
β Source: BBC News (https://www.bbc.com/news/articles/c3wlqy6695do)