Castro's natural successor is his younger brother Raúl, 76, the chief of the Armed Forces, who has served as the acting president since his brother fell ill in July 2006.

Given the age of Raúl Castro and his dislike for the political limelight some analysts speculate that the job could go to Carlos Lage, 56, the Vice-President, who represents a new generation of Cuban politicians.

Fidel Castro hinted as much in his letter of resignation yesterday. “There is also the intermediate generation that learnt together with us the basics of the complex and almost unattainable art of organising and leading a revolution,” he wrote.

Mr Lage exercises wide control over government administration, much like a prime minister, and often represents Cuba abroad. He has earned a place in the past two decades as one of the most trusted advisers of Mr Castro. When the President ceded power in July he gave Mr Lage sole responsibility for energy and the economy.

A paediatrician by training, Mr Lage is from a family with excellent revolutionary credentials. The student leader become a member of the Cuban Communist Party's central committee in 1991.

Although not a charismatic figure he is respected as a listener and capable administrator. He became indispensable after the collapse of the Soviet Union when the country struggled to adapt to the loss of trade with Moscow.

Behind the scenes the new leader can depend on a trusted core of experienced Communist Party officials. Mr Castro is expected to take a back seat, limiting himself to what some have called commentator-in-chief, a reference to the series of essays, or “reflections”, that he has published in state newspapers since his illness.

If Raúl Castro is chosen as president he could not have been better groomed for the job. “Raúl has been in the background all these years,” Domingo Amuchastegui, a former Cuban intelligence analyst now living in Miami, said. “But he is the man who has implemented many of Fidel's policies. He's an army guy. He's organised and he's efficient.”

Whoever takes over is expected to take the country on a different course, analysts say, including possible economic reforms as well as overtures for a normalisation of relations with the United States. In the short term, unity will be the key. “There will be no division among Cuban revolutionaries,” Mr Lage said at a 80th birthday celebration for Mr Castro in 2006. “There will be no ambitions, no egos.”

David Adams has been reporting on Cuba for 20 years

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