Cuban President Fidel Castro has provisionally handed over power to his brother to undergo surgery on his stomach.
He delegated his posts as first secretary of the ruling Communist Party, commander in chief of the armed forces and president of the executive council of state to Raul Castro (75), his designated successor.
President Castro, who has led Cuba since his supporters overthrew dictator Fulgencio Batista in 1959, said he over-exerted himself this month on a trip to a summit of South American leaders and during celebrations of his 1953 assault on a military garrison.
President Fidel Castro's brother Raul
"This caused an acute intestinal crisis with sustained bleeding," said the statement signed by Mr Castro and read out by an aide.
Mr Castro asked the country to postpone celebrations of his 80th birthday, on August 13th, until December 2nd of this year.
The December date marks the 50th anniversary of when he landed a small boatload of rebels in Cuba in 1956 to launch a rebellion that eventually won power in 1959.
Street celebrations immediately erupted in Miami, a hotbed of opposition to Mr Castro's communist government where news of his delegation of power was greeted as a signal of his imminent demise.
Mr Castro's health has been an issue since he fainted during a speech in 2001, raising uncertainty over the future of Cuba. His pace has slowed since he stumbled after a speech in October 2004, fracturing a knee and an arm.
Mr Castro said he was delegating power to his brother because Cuba was "under threat from the US government."
President Bush has tightened enforcement of sanctions on Cuba and increased funding of Cuba's small and repressed dissident movement.
Raul Castro said in June that the Communist Party will govern Cuba and maintain the island's socialist society when Fidel is no longer around.
US officials said today they were monitoring the situation in Cuba and were cautious about the future.