British intelligence chiefs probably knew it was a fake but released it because they were happy for Labour to be destabilised by its inflammatory language. A Foreign Office official who had incurred heavy debts through currency speculation may have sold a copy to the Daily Mail, whose publication of it caused such a furore.
These are the conclusions reached by Gill Bennett, the Foreign Office's chief historian, who has been given access to British and Soviet files to try to solve the mystery.
The letter was addressed to the Communist Party of Great Britain, purportedly from Grigory Zinoviev, the man in charge of Soviet efforts to encourage Communist activity abroad.
It called on British Communists to mobilise "sympathetic forces" in the Labour Party and badly damaged Labour when it was published in the Mail, whose editor was Thomas Marlowe, although it was a collapse in the Liberal vote that lost Labour the election.
Ms Bennett said she believes the letter was the work of White Russian emigres - tsarists who opposed the Bolsheviks - who were angry that Britain's first Labour government had signed a treaty with the Soviet Union and was making it a loan.
The White Russians had the means - a forging circle - and the contacts in the West and in Moscow to be able to produce the letter and circulate it.
Ms Bennett said she did not think the full answer to the puzzle was knowable today: "The story remains incomplete ... the Zinoviev Letter remains, as before, a most extraordinary and mysterious business."
But her inquiries, including a visit to Moscow, led her to dismiss speculation that the letter was part of a plot by British intelligence staff to discredit both the Bolsheviks and the Labour government. Neither was it likely that Zinoviev wrote the letter, she said. Russia wanted the British loan and was therefore holding back on fomenting discontent.
However, when the letter emerged in London, intelligence staff, including Desmond Morton, later Churchill's close aide, and Joseph Ball, who later worked for Conservative Central Office, may have decided to use it for their own political means. The Tories certainly capitalised on the affair, raising the possibility that they leaked it to the Mail.
Alternatively, the intelligence services may have passed it to the paper themselves. Another possibility was that J D (Don) Gregory, head of the Foreign Office's Northern Department, sold a copy to pay debts he had accumulated through currency speculation with an "extravagant" and "capricious" married woman, Aminta Dyne. His colleagues believed she was his lover.
The intelligence services made no efforts to authenticate the document when it arrived from their agent in Riga, but it was distributed to the Foreign Office, Scotland Yard and the War Office with a note: "The authenticity of the document is undoubted."
Ms Bennett said yesterday: "I have my doubts about whether (Morton) thought it was genuine but he treated it as if it was." Ms Bennett's report was commissioned by Robin Cook, the Foreign Secretary, after questions were asked in the Commons last year, prompted by a book by the espionage writer Nigel West.
Ms Bennett said the Russians had been surprised that the Zinoviev affair still had so much resonance in Britain today. She explained the appeal like this: "It's not quite sex, lies and videotape but it's certainly sex and spies. It's a fairly potent combination."
Soviet propaganda chief and purported author of letter to British Communist Party
James Ramsay MacDonald
Britain's first Labour prime minister, was severely embarrassed by the letter
`Mail' editor published letter under headline `Civil War Plot By Socialists' Masters'
Foreign Secretary ordered the inquiry and sanctioned use of security service filesReuse content