Sir Simon Milton, Boris Johnson's Chief of Staff, has died age 49 after a short illness. Milton led Westminster Council for eight years before his appointment as Deputy Mayor For Policy and Planning in 2008.
He was a steady hand on the tiller after a number of Johnson's senior staff resigned in the early days of his Mayoralty.
Boris Johnson said: "He was a wonderful colleague and friend and will be much missed by everyone who was involved in public life in this city."
Guardian journalist Dave Hill is collecting together tributes to Sir Simon, adding that his 'vast knowledge of local government and importance to the present mayoralty were disputed by no one'.
The BBC has more.