The Zinoviev letter - one of the greatest British political scandals of this century - was forged by a MI6 agent's source and almost certainly leaked by MI6 or MI5 officers to the Conservative Party, according to an official report published today.

New light on the scandal which triggered the fall of the first Labour government in 1924 is shed in a study by Gill Bennett, chief historian at the Foreign Office, commissioned by Robin Cook.

It points the finger at Desmond Morton, an MI6 officer and close friend of Churchill who appointed him personal assistant during the second world war, and at Major Joseph Ball, an MI5 officer who joined Conservative Central Office in 1926.

The exact route of the forged letter to the Daily Mail will never be known, Ms Bennett said yesterday. There were other possible conduits, including Stewart Menzies, a future head of MI6 who, according to MI6 files, admitted sending a copy to the Mail.

The letter, purported to be from Grigori Zinoviev, president of the Comintern, the internal communist organisation, called on British communists to mobilise "sympathetic forces" in the Labour Party to support an Anglo-Soviet treaty (including a loan to the Bolshevik government) and to encourage "agitation-propaganda" in the armed forces.

On October 25, 1924, four days before the election, the Mail splashed headlines across its front page claiming: Civil War Plot by Socialists' Masters: Moscow Orders To Our Reds; Great Plot Disclosed. Labour lost by a landslide.

Ms Bennett said the letter "probably was leaked from SIS [the Secret Intelligence Service, commonly known as MI6] by somebody to the Conservative Party Central Office". She named Major Ball and Mr Morton, who was responsible for assessing agents' reports.

"I have my doubts as to whether he thought it was genuine but [Morton] treated it as if it was," she said. She described MI6 as being at the centre of the scandal, although it was impossible to say whether the head of MI6, Admiral Hugh Sinclair, was involved.

She said there was no evidence of a conspiracy in what she called "the institutional sense". The security and intelligence community at the time consisted of a "very, very incestuous circle, an elite network" who went to school together. Their allegiances, she says in her report, "lay firmly in the Conservative camp".

Ms Bennett had full access to secret files held by MI6 (some have been destroyed) and MI5. She also saw Soviet archives in Moscow before writing her 128-page study. The files show the forged Zinoviev letter was widely circulated, including to senior army officers, to inflict maximum damage on the Labour government.

She found no evidence to identify the name of the forger. She said the letter - sent to MI6 from one of its agents in the Latvian capital, Riga - was written as a result of a campaign orchestrated by White Russians who had good contacts in London who were strongly opposed to the Anglo-Soviet treaty.

The report says there is no hard evidence that MI6 agents in Riga were directly responsible - though it is known they had close contacts with White Russians - or that the letter was commissioned in response to British intelligence services' "uneasiness about its prospects under a re-elected Labour government".

However, if Ms Bennett is right in her suggestion that MI6 chiefs did not set up the forgery, her report makes clear that MI6 deceived the Foreign Office by asserting it did know who the source was - a deception it used to insist, wrongly, that the Zinoviev letter was genuine.

Ms Bennett says it is wrong to conclude the scandal brought down Ramsay Macdonald's government which had already lost a confidence vote and Liberal support on which it depended was disappearing. The Labour vote in the election actually increased by a million.

"In electoral terms", she says, "the impact of the Zinoviev letter on Labour was more psychological than measurable".

The Zinoviev letter was not the only attempt by the security and intelligence services to destabilised a Labour government. Peter Wright, a former MI5 officer, showed in Spycatcher how elements in his agency worked against the Wilson government in the 1970s.