The Zinoviev letter was a fake document published and sensationalized by the British Daily Mail newspaper four days before the general election of October 1924. The letter purported to be a directive from Grigory Zinoviev, the head of the Communist International (Comintern) in Moscow, to the Communist Party of Great Britain, ordering it to engage in seditious activities. It claimed that the resumption of UK diplomatic relations with the Soviet Union under a Labour government would radicalise the British working class. The right-wing press depicted these falsified remarks as a grave foreign interference in and subversion of British politics, and as a result it offended some British voters, turning them against the Labour Party.

The letter seemed authentic to some at the time, but historians now agree it was a forgery. The letter aided the Conservative Party by hastening the collapse of the Liberal Party vote that produced a Conservative landslide. A. J. P. Taylor argued that the most important impact was on the mindset of Labourites, who in his estimation for years afterwards blamed foul play for their defeat, thereby misunderstanding the political forces at work and postponing what he regarded as necessary reforms in the Labour Party. Many others have disagreed and cited the letter as a truly decisive contribution to the party's loss of seats and fall from government.