Image Fidel Castro at a 2005 speech in Havana, Cuba. Credit... Jorge Rey/Associated Press

But the announcement puts Raúl Castro in position to be anointed as the Cuban head of state when the National Assembly meets on Sunday, cementing the power structure that has run the country since Mr. Castro fell ill.

However, Mr. Castro’s unexpected announcement left it unclear what role other high-level government ministers  among them the vice president, Carlos Lage Davila, and the foreign minister, Felipe Perez Roque  would play in the new government.

Mr. Castro also made it clear he is not fading into the sunset but pledged to continue to be a force in Cuban politics through his writings, just as he has over the last year and a half. “I am not saying goodbye to you,” he wrote. “I only wish to fight as a soldier of ideas.”

That statement raised the possibility little would change after Sunday’s vote, that Cuba will continue to be ruled in essence by two presidents, with Raúl Castro on stage while Fidel Castro lurks in the wings. At times over the last year and a half, the current government has seemed paralyzed when the two men disagree. In Washington, John D. Negroponte, deputy secretary of state, said it was unlikely that the United States would lift its trade embargo on Cuba, Reuters reported.

Image Fidel Castro, left, with his brother, Raul, in 2006. Credit... Cristobal Herrera/Associated Press

For his part, Mr. Castro has sent several signals in recent months that it was time for a younger generation to take the helm. He said in December, for example, “My primary duty is not to weld myself to offices, much less obstruct the path of younger people.”