The "Zinoviev Letter" is thought to have been instrumental in the Conservative Party's victory in the United Kingdom general election, 1924, which ended the country's first Labour government. The Conservative Party is the largest political party on the right-of-centre in the United Kingdom. ... The 1924 UK general election was held on 29th October 1924. ... The Labour Party is a centre-left or social democratic political party in Britain (see British politics), and one of the United Kingdoms three main political parties. ...

In 1999, an official enquiry (see Telegraph, 5 Feb. 1999) finally determined that the latter had probably been a concoction by elements of the SIS based in Riga to help the Conservatives defeat Labour in the 1924 election.

Allegedly addressed from Grigori Zinoviev, president of the presidium of the Executive Committee of the Communist International (Comintern), to the Central Committee of the Communist Party of Great Britain, the letter purported to advocate intensified Communist agitation in Britain, not least in the armed forces. Grigory Yevseevich Zinoviev (Григо́рий Евсе́евич Зино́вьев, real name Ovsel Gershon Aronov Radomyslsky (Радомысльский), also known as Hirsch Apfelbaum), (September 23 [September 11, Old Style], 1883 - August 25, 1936) was a Bolshevik revolutionary and a Soviet Communist politician. ... The first edition of Communist International, journal of the Comintern published in Moscow and Petrograd (now Saint Petersburg) in May 1919. ... The Communist Party of Great Britain (CPGB) was a political party in the United Kingdom, which existed from 1920 to 1991. ...

Published in the conservative British Daily Mail newspaper four days before the election, the letter came at a sensitive time also in relations between Britain and the Soviet Union, owing to Conservative opposition to the forthcoming parliamentary ratification of the Anglo-Soviet trade agreement of August 8. The Daily Mail is a British newspaper, currently a tabloid, first published in 1896. ...

Dated September 15, 1924, the letter is generally thought to be a forgery. Although much of its content otherwise persuasively echoes Comintern vocabulary, the letter contains errors (such as "Executive Committee, Third Communist International" - a nonsensical title) which led many even at the time to denounce it as a hoax.

A particularly damaging section of the document identified the normalisation of inter-governmental relations under the Anglo-Soviet agreement as an opportune moment for increased Soviet propaganda activity within the British Labour movement:

"A settlement of relations between the two countries will assist in the revolutionising of the international and British proletariat not less than a successful rising in any of the working districts of England, as the establishment of close contact between the British and Russian proletariat, the exchange of delegations and workers, etc. will make it possible for us to extend and develop the propaganda of ideas of Leninism in England and the Colonies."

British Labour prime minister Ramsay MacDonald's attempts to cast doubt on the letter's authenticity were hampered by its widespread acceptance among government officials. MacDonald "felt like a man sewn in a sack and thrown into the sea", he told his Cabinet on October 31 as they prepared to leave office. The Right Honourable James Ramsay MacDonald, PC (12 October 1866–9 November 1937), British politician, was twice Prime Minister of the United Kingdom. ...

The Soviet authorities for their part were prevented by poor communication with the Moscow-based ECCI from delivering the immediate unequivocal refutation of the letter required following a British Foreign Office protest note of October 24: not until November 17 did the ECCI even discuss the matter. On November 21 Britain's new Conservative government repudiated the unratified treaty.

An 11-month study by British Foreign Office chief historian Gill Bennett, undertaken with the assistance of Russian archivists, concluded (January 1999) that the document was probably forged at the behest of "White [i.e. anti-communist] Russian intelligence services" to "derail the treaties and damage the Labour government”. 'Head office' British intelligence responsibility for the letter was "inherently unlikely" as it "implied a degree of cohesion and control, not to mention political will, which simply did not exist".

Reputedly 1950s British prime minister Anthony Eden believed that the letter was the work of MI5. The Right Honourable Robert Anthony Eden, 1st Earl of Avon, KG, MC, PC (June 12, 1897– January 14, 1977), British politician, was Foreign Secretary during World War II and Prime Minister of the United Kingdom during the 1950s. ... Current MI5 headquarters in Thames House, London MI5, officially called the Security Service, is a British counter-intelligence and security agency. ...

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Zinoviev Letter