This is the story of one of the most enduring conspiracy theories in British politics, an intrigue that still has resonance almost a century later: the Zinoviev Letter of 1924. Almost certainly a forgery, no original has ever been traced, and even if genuine it was probably Soviet 'fake news'. Despite this, the Letter still haunts British politics nearly a century after it was written; it was the subject of major Whitehall investigations in the 1960s and 1990s, and cropped up in the media as recently as during the Referendum campaign and the 2017 general election.The Letter, encouraging the British proletariat to greater revolutionary fervour, was apparently sent by Grigori Zinoviev, head of the Bolshevik propaganda organization, to the British Communist Party in September 1924. Sent to London through British Secret Intelligence Service channels, it arrived during the general election campaign and was leaked to the press. The Letter's publication by the Daily Mail on 25 October 1924 just before the General Election humiliated the first ever British Labour government, headed by Ramsay MacDonald, when its political opponents used it to create a 'Red Scare' in the media. Labour blamed the Letter for its defeat, insisting there had been a right-wing Establishment conspiracy, and many in the Labour Party have never forgotten it.The Zinoviev Letter has long been a symbol of political dirty tricks and what we would now call 'fake news'. But it is also a gripping historical detective story of spies and secrets, fraud and forgery, international subversion and the nascent global conflict between communism and capitalism.