Spain's former King Juan Carlos shares how he killed his brother when they were teenagers

Juan Carlos told readers: "I didn't like to talk about it, and this is the first time I do."

In the book, which was published in France under the title "Juan Carlos I d'Espagne: Reconciliation", the former royal looks to improve relations with his son and the King of Spain, Felipe VI.

Revisiting the incident that occurred in his childhood, Juan Carlos said he and his brother were "playing" with a pistol at their family's home in Portugal.

The 87-year-old said: "I will not recover from this tragedy. Its gravity will accompany me forever."

Juan Carlos explained that the pistol's magazine had been removed, so he thought it posed no danger.

He wrote: "We had taken out the magazine. We had no idea there was a bullet left in the chamber.

"A shot was fired into the air, the bullet ricocheted and struck my brother squarely in the forehead. He died in our father's arms."

No judicial inquiry into the circumstances surrounding the firearms accident took place at the time.

The boy's father proceeded to cover the body of Alfonso in a Spanish flag and later threw the pistol into the sea.

Juan Carlos was sent back to his austere military academy, as his relationship with his father, the Count of Barcelona, fell apart.

Reflecting on the event, the former King wrote: "There is a before and an after.

"It is still difficult for me to speak of it, and I think of it every day... I miss him; I wish I could have him by my side and talk with him.

"I lost a friend, a confidant. He left me with an immense emptiness. Without his death, my life would have been less dark, less unhappy."

The book, which is divided into seven parts, will be published in Spanish in December, coinciding with the 50th anniversary of the death of Franco and the return to a monarchy.

French author and historian Laurence Debray, who spent two years interviewing Juan Carlos, said the revelations were "quite explicit".

Inside the memoir, he denied that he had an affair with Princess Diana, describing her as "cold and taciturn".

Royal watchers speculated whether Juan Carlos seduced the princess at the Marivent Palace, the Spanish royal family's summer home.

Juan Carlos is believed to have had relations with multiple women outside of his marriage to the then Queen Sofia, including affairs with numerous Spanish actresses.

Diana never discussed the rumours publicly, although she was believed to have found him "a little too attentive".

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