General Franco went on to rule Spain for 39 years

The documents also reveal that MI6 had its own man inside the Soviet Communist Congress of 1920.

What the mole reported was later 'planted' in The Times of London on the orders of Winston Churchill, who was then Minister for War in the Lloyd George coalition government.

Details of the Franco plot were devised by Nikolai Yezhov, head of OGPU, the Soviet secret police, according to an MI5 memo by Soviet defector General Walter Krivitsky.

'Fanatical'

Yezhov instructed another agent, Paul Hardt, to find an Englishman to carry out the job.

"He did in fact contact and sent to Spain a young Englishman, a journalist of good family, an idealist and a fanatical anti-Nazi," said Gen Krivitsky in his memo.

The words "prob Philby" were later written next to this passage in the margin of the document by an intelligence officer.

The spy reported Lenin's message to the secret congress

Philby was a journalist covering the Spanish Civil War for The Times when the alleged plan was hatched.

The memo continues: "Before the plan matured, Hardt himself was recalled to Moscow and disappeared."

General Franco's forces went on to win the war and the dictator ruled Spain for 39 years.

Philby, head of the notorious Cambridge spy ring which leaked sensitive state secrets to the USSR, fled to Russia in the 1960s after the extent of his deceit was unmasked.

Lenin's message

Disclosures about the Moscow mole are made in copies of MI6 papers about the late Soviet leader Vladimir Ilyich Lenin, compiled by the domestic security service MI5.

The Third International in Moscow in September 1920 was a critical moment for Lenin and the fledgling Soviet government, mapping a new global strategy while preparing to deal with its own post-war problems.

The Daily Herald featured Nicolai Yezhov, former head of the Soviet Secret Service

Delegations of Communist parties from all over Europe came to Moscow - even a three-man delegation from Switzerland.

It was one of the Swiss delegation, Pastor Humbert Droz, from La Chaux de Fonds, whose report to MI6 in London appears on the file.

He gives a concise and detailed account of Lenin's message to the secret congress: