From The Independent in 1999:

Official: Zinoviev letter was forged By Louise Jury Wednesday 3 February 1999 THE ZINOVIEV Letter, which was blamed for Labour losing the 1924 general election, was almost definitely a forgery, an inquiry has concluded. 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. … 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.”

I’m disappointed that the researcher didn’t point the finger at Maundy Gregory, the International Man of Mystery involved in various skullduggery for David Lloyd George, such as selling knighthoods for 10,000 pounds. Gregory was long rumored by leftists to have forged the Zinoviev Letter with the help of Sidney Reilly, Ace of Spies.

Dashiell Hammett is said to have based the character of The Fat Man in The Maltese Falcon on Gregory, although I can’t find much explanation of how Hammett knew of him. The detective probably read about Gregory in leftist newspapers.

Richard Davenport-Hines described Gregory as being “short, paunchy, bald, rubicund, monocled, and epicene.” Davenport-Hines added: “He wore ostentatious jewelry, including a green scarab ring he claimed had been Wilde’s, and used to fidget with a rose-coloured diamond carried in his waistcoat pocket which supposedly had belonged to Catherine the Great. His manner was grandiose, mysterious, watchful, and confidential.”

Gutman the Fat Man, Sidney Greenstreet’s first role in a Hollywood movie, made such an impact on audiences in the 1940s that the Nagasaki atomic bomb was named Fat Man.

British secret agent Sidney Reilly is another interesting character in pop culture.

He was a model for James Bond.

Reilly was played by Sam Neill in the 1983 miniseries Reilly: Ace of Spies.

From Wikipedia:

The origins, identities and activities of Sidney Reilly have befuddled researchers and intelligence agencies for more than 100 years, and much of his purported life and many of his notorious exploits should be cautiously examined. Reilly himself told several versions of his origins to confuse and mislead investigators. He claimed to be the son of an Irish merchant seaman, an Irish clergyman, and an aristocratic landowner and habitué of the Imperial court of Tsar Alexander III of Russia. According to the Ukrainian newspaper Segodnya, he was born Zigmund Markovich Rozenblum (Rosenblum) on 24 March 1874 in Odessa, then a Black Sea port of the Russian Empire. … The endeavour to depose the Bolshevik Government and assassinate Vladimir Lenin is considered by biographers to be Reilly’s most daring scheme.[3] The Lockhart Plot, or more accurately the Reilly Plot, has sparked debate over the years: Did the Allies launch a clandestine operation to overthrow the Bolsheviks? If so, did the Cheka uncover the plot at the eleventh hour or had they unmasked the conspirators from the outset? Some historians have suggested that the Cheka orchestrated the conspiracy from beginning to end and possibly that Reilly was a Bolshevik agent provocateur. … Reilly told various tales about his espionage deeds and adventurous exploits. According to Reilly, he earned and lost several fortunes in his lifetime and had many wives and mistresses. He claimed that: – In the Second Boer War he disguised himself as a Russian arms merchant to spy on Dutch weapons shipments to the Boers. – He procured Persian oil concessions for the British Admiralty, the so-called D’Arcy Affair. – In the disguise of a timber company owner, he gathered information on the Russian military presence in Port Arthur, Manchuria, and reported to the Kempeitai, the Japanese secret police. – He spied on the Krupp armaments plant in Germany. – He volunteered for the Royal Flying Corps in Canada at the start of the First World War. – He seduced the wife of a Russian minister to obtain information about German weapons shipments to Russia. – During the First World War, he donned a German officer’s uniform and attended a German Army High Command meeting. – He saved British diplomats in Brazil. – He attempted, but failed, to engineer the downfall of the Russian Bolshevik government.

On the other hand, maybe Reilly was mostly just a blowhard.

According to Cook, Reilly was more of a con artist. Reilly claimed to have been employed by the British Secret Intelligence Service since the 1890s, but he did not volunteer his services nor was he accepted as an agent until 15 March 1918, and was effectively fired in 1921 because of his tendency to be a rogue operative. Nevertheless, Reilly had been a renowned operative for Scotland Yard’s Special Branch and the Secret Service Bureau, which were the early forerunners of the British intelligence community. … Author Michael Kettle has claimed in Sidney Reilly: The True Story of the World’s Greatest Spy (pg. 121) that despite having been fired by SIS, Reilly possibly was involved with Sir Stewart Graham Menzies in the forging of The Zinoviev Letter in 1924. …

In any case, Reilly died bravely:

In September 1925, undercover agents of the OGPU, the intelligence successor of the Cheka, lured Reilly to Bolshevik Russia, ostensibly to meet the supposed anti-Communist organization The Trust—in reality, an OGPU deception existing under the code name Operation Trust.

The Trust was a fake anti-Bolshevik resistance operation concocted by the Bolsheviks to lure White Russians back to Russia on (not so) secret missions.

After Reilly crossed the Finnish border, the Soviets captured, transported, and interrogated him at Lubyanka Prison. In his book The Gulag Archipelago, Alexandr Solzhenitsyn says that Richard Ohola, a Finnish Red Guard, was “a participant in the capture of British agent Sidney Reilly.” … According to British intelligence documents released in 2000, Reilly was executed in a forest near Moscow on Wednesday 5 November 1925. … Gudz also confirmed that the order to kill Reilly came from Stalin directly.

Or maybe not …

After Reilly’s death there were various rumours about his survival. Some, for example, speculated that Reilly had defected and became an adviser to Soviet intelligence.

In summary, Russian stuff can be complicated.