Revolutionary "Che Guevara" was viewed by British officials as the second most important figure in Cuba after Fidel Castro, newly released documents show.
Che Guevara is believed to have been killed by the Bolivian Army
Ernesto Guevara Serna was listed among 61 people in the 1967 Foreign Office report, Leading Personalities in Cuba.
Havana embassy staff wrote: "He is an able and hardworking man who was perhaps the most competent and the clearest-headed of the inner circle."
The papers were in a selection released by the National Archives.
Officials said Guevara could "show himself cultured and soft-spoken as well as cold and contemptuous."
"This bearded Argentinean, with his Irish charm and his inevitable military fatigue uniform, has exercised considerable fascination over many men and women," they added.
"At the beginning of the revolution it was Guevara who radically altered Cuba's traditional trading pattern, which his missions to uncommitted and Communist countries made possible.
"Until 1964 he was perhaps the most influential figure in Cuba after Fidel Castro."
But they said Fidel Castro became less inclined to take Che Guevara's advice after some of his ideas - such as actively spreading revolution through Latin America - met setbacks.
Major Guevara's revolutionary zeal is said to have been fuelled by a motorcycle tour of Chile and Peru and a canoe trip along the upper Amazon, his diary of which has been turned into a film The Motorcycle Diaries.
Che Guevara has become an icon in Cuba and parts of Latin America
It was during a subsequent trip to Mexico that he is believed to have first met Fidel Castro, joining the fight against Fulgencio Batista's government, which was overthrown in 1959.
Officials wrote of Major Guevara's "mysterious departure" from Cuba in 1965 after returning from a trip to Africa, and questioned whether he was still alive.
"Not until October 1965 did Castro make public the famous letter allegedly written in April, in which Guevara indicated that he had left Cuba to pursue the revolutionary cause elsewhere and in which he emphatically endorsed Castro's virtual infallibility.
"Since then Guevara has been integrated into the revolutionary hagiography, but nothing definite has been heard of him", they said, adding there were rumours he had died.
Che Guevara is now believed to have been killed by the Bolivian Army the month after Leading Personalities of Cuba was written.
Descriptions of President Fidel Castro and his brother and head of Cuba's armed forces, Raul Castro Ruz, have been cut from the released documents.
However, other Cuban personalities listed in the contents were described in detail - some seemingly with a more personal than political focus.
Raul Castro Ruz' wife, Vilma Espin de Castro, was among them: "A strikingly handsome and even attractive woman, who uses much more make-up and other aids than is the revolutionary custom and manages to make even her uniforms smart and feminine".
Of another official the embassy wrote: "He does not give the impression of being very intelligent or of cutting much ice among his brighter colleagues: but they will probably tolerate him because of his impeccably humble background and friendship with Fidel."