Castro must be re-elected to Cuba's national assembly in order to continue as head of the council of state, Cuba's main governing body.
The parliamentary elections are due to be held on January 20.
'Lucid' leader
Castro's letter comes a day after he met Luiz Inacio Lula Da Silva, the Brazilian president, who said that he found the Cuban leader to be in good health and "as lucid as he as ever been".
"I think Fidel is ready to take over his historic political role in this globalised world, in humanity," Silva said after leaving Cuba late on Tuesday.
Castro suggested in a letter last month he would not cling to power, nor oppose a younger generation of Cuban leaders.
However he has not indicated when, or if, he will step aside permanently.
Castro, who took power in a 1959 revolution, remains South America's only communist leader.
A long-term foe of the United States, which has imposed a crippling economic and political embargo on the country, he has survived several attempts on his life and outlasted nine US presidents.
He has ruled Cuba for so long that many of his fellow Cuban have known no other leader.