A biography of Spain's Queen Sofia portrays King Juan Carlos as a womaniser who tried to seduce Princess Diana.
The book, The Solitude of the Queen, paints a sad picture of the king and queen of Spain's private relationship.
It describes Juan Carlos as "a professional seducer" who had several affairs.
According to the book, Juan Carlos even tried to court Princess Diana during a holiday with Prince Charles in Majorca in the 1980s. The British and Spanish royal couples were considered close friends.
The book will feed speculation that the princess had a fling with Juan Carlos between 1986 and 1987.
Pilar Eyre's biography claims that Queen Sofia last slept in the marital bed in 1976. She stayed in the marriage only out of a sense of duty, said Eyre.
The book details some of Juan Carlos's alleged affairs and includes claims that he was seeing a 25-year-old German translator last year.
"Queen Sofia is a woman betrayed and hurt with a married life that has been a real tragedy. The king's closest friends I have spoken to say they don't like her," Eyre said in Spanish gossip magazine Vanitatis.
The author has allegedly been banned from working on Spanish TV's Channel Telecinco since the book was published.