Never close as children, Juanita Castro and her older brother Fidel shared none of the customary warmth of a large Cuban family. Their differences were to prove telling.

Within two years of the Cuban revolutionary’s victory over the dictator Fulgencio Batista in 1959, Ms Castro had turned traitor, betraying her brother to his arch enemy, America’s Central Intelligence Agency.

For three years, as the communist regime was rocked by the Bay of Pigs invasion and the missile crisis, Juanita Castro spied for the US secret service.

Now living in Miami, Ms Castro disclosed for the first time yesterday how she passed secrets about the former Cuban leader to the CIA between 1961 and 1964, before going into US exile.

Ms Castro, 76, who has not spoken to Fidel or his younger brother, Raúl, the current Cuban President, in more than 40 years, made the disclosure days before the publication of her memoirs, Fidel and Raúl, My Brothers, the Secret History. In an interview with the Spanish-language television channel Univision Noticias 23 in Miami, Ms Castro told how she supported initially the 1959 insurgency that toppled the Batista regime.

In 1958, with revolution in the air, Ms Castro, then 24, travelled to the US from her home in Havana to raise funds for her brothers, who were fighting a guerrilla war in the Sierra Maestra mountains. She had stayed with the family in Havana, sending food and trying to raise support, while Fidel and Raúl led the assault on Batista’s forces.

But after Fidel took power, Ms Castro, who is seven years his junior, became disillusioned as he executed opponents and propelled the country towards communism. “I began to become disenchanted when I saw so much injustice,” she said.

Ms Castro, one of seven siblings, said that she began to shelter enemies of her brother’s administration at her home in Havana.

“My situation became delicate because of my activity against the regime,” she said. “Fidel stopped coming to our house because he complained we were protecting what he called ‘worms’ and he did not agree.”

She was recruited to work with the CIA through an approach by the wife of the Brazilian Ambassador to Cuba at the time. “They wanted to talk to me because they had interesting things to tell me, and interesting things to ask me, such as if I was willing to take the risk, if I was ready to listen to them. I was rather shocked but, anyway, I said yes,” she said.

Maria Antonieta Collins, a Mexican journalist with whom Ms Castro wrote her book, said: “In this way began a long relationship with the arch enemy of Fidel Castro, the Central Intelligence Agency ... From 1961 to 1964, at the risk of her own life, the work of Juanita Castro was to save the lives of her compatriots long before she left for exile in Miami.”

However, Ms Castro’s claims leave many questions unanswered. Did the information that she passed to the CIA, for instance, help in any of the supposed 638 unsuccessful assassination attempts on her brother’s life?

Since leaving Cuba, Ms Castro has remained a critic of its rulers, stating that her brother had betrayed the principles he originally espoused.

Before retiring in 2006, she lived quietly, running a pharmacy in Miami. Her only brush with fame was when Andy Warhol made a film about her life in 1965, as a satire on the Cuban revolution. She last spoke to Fidel in 1963 when their mother, Lina Ruz González, died of a heart attack. She spoke to Raúl days before going into exile in July 1964.

The Associated Press reported in 1964 that she worked “voluntarily” for the US secret service for four years, just before she left Cuba. Philip Agee, the CIA agent, also wrote in his book Inside The Company: CIA Diary, published in 1975, that Ms Castro was a “propaganda agent” for the US.

The Castros were a middle-class family noted for their lack of warmth, according to those who knew them during the 1930s. Angel Castro, a businessman in the sugar trade, was described as hard and avaricious.

Fidel Castro, 83, who ruled the country for nearly half a century, was forced by ill health to hand over the presidency to Raúl, 78, last year.

Times Recommends

• UN may halt 400-year-old 'freedom of the seas'
• Where the Rosetta belongs can’t be set in stone
• Gas deal on hold, but Ukraine may pay in the end