A British spy and close friend of Winston Churchill was deeply implicated in the Zinoviev Letter, the most notorious political forgery in British history.

The publication of a letter purporting to be from Soviet officials four days before the 1924 general election helped to sweep Ramsay MacDonald's government from power. But the correspondence mobilising "sympathetic forces" in Labour was later found to be a fake.

Now a new official history, based on access to closed intelligence files, suggests the document was a "dirty tricks" operation by the Secret Intelligence Service (SIS), overseen by Major Desmond Morton. Morton, a First World War hero whom Churchill befriended in the trenches, became an SIS officer on his recommendation and went on to become the war leader's "spymaster". In the early years of the Second World War, he was a trusted fixer within the inner circle of Churchill's bunker.

His role was recently lauded in the film The Gathering Storm, in which Morton, played by Jim Broadbent, is shown leaking intelligence on German preparations for war to Churchill during his "wilderness years".

The truth, however, appears to be rather less heroic. Gill Bennett, until recently chief historian at the Foreign Office, has found that Morton was "centre stage from the beginning" of one of Britain's worst political scandals.

It was Morton who first received the letter, purporting to be from Grigori Zinoviev, president of the Comintern, the internal communist organisation, from an agent in Riga, Latvia. It called on British Communists to mobilise the Labour Party to support an Anglo-Soviet treaty.