This article examines in detail the sustained campaign of character assassination run by Mercurius Britanicus against Charles I. It illustrates the substance, style and provenance of Britanicus's arguments; it then assesses the newsbook's attacks in their chronological context.The article argues that Britanicus worked closely with the parliamentary war party grandees to discredit Charles publicly, to divide royalist opinion by revealing his weaknesses and mistakes, and to pressurize him into making peace on the radicals' terms. Although Britanicus stopped short of calling outright for Charles's deposition, this was the logical conclusion to be drawn from the newsbook's comments.