The ex-king of Spain, Juan Carlos, was leading a double life and having an affair with a German aristocrat for the last ten years of his reign, according to a new book that claims they were 'a couple, pure and simple'. The bombshell allegation is that Juan Carlos was romancing Corinna zu Sayn-Wittgenstein and was so smitten with her that he even contemplated divorcing his wife, Sofia, according to Final De Partida – or End Game – by Ana Romero, which sold out within 24 hours. Romero, a former royal correspondent for the El Mundo daily, said at the book's launch in Madrid: 'What do you see (in the book)? Well, the double life which Juan Carlos led during a long time, a double life which obviously has not gone well.' Scroll down for video . The former King Juan Carlos greets Corinna Sayn-Wittgenstein in at an awards ceremony in Barcelona in May 2006:  A new book claims that they have been lovers for a decade . Glamorous: The aristocrat is 27 years younger than the former Spanish king. Their affair is said to have ended in 2014 . The author's claims are based on 100 interviews with anonymous sources. She said that the affair ended in 2014 when Juan Carlos abdicated in favour of his son, Felipe. He married former Queen Sofia in 1962 and became king in 1975. Romero added: '[Final De Partida]  is a chronicle of the end of an era, when Spaniards turned their backs on Juan Carlos. The first print run of 25,000 copies nearly sold out on Wednesday less than 24 hours since the book hit stores and its publishers have already ordered a second print run. The 77-year-old former king's friendship with Sayn-Wittgenstein, 50, who is divorced from the German prince whose name she bears, emerged in 2012 when Juan Carlos was flown home from Botswana on a private jet after suffering an injury while on an expensive elephant-hunting holiday. Juan Carlos, pictured here with Queen Sofia in May 2014, was said to be so smitten with Sayn-Wittgenstein that he considered a divorce . The German woman, who is 27 years younger than Juan Carlos, was among those accompanying the monarch on the trip, which sparked outrage at a time when Spain was grappling with record unemployment and risked needing an international bailout. Juan Carlos, who won respect for his role in guiding Spain's transition to democracy after the death of longtime dictator General Francisco Franco in 1975, made an unprecedented apology for his behaviour but his popularity took a serious hit. Romero said the uproar over the hunting trip, combined with a string of health problems and a corruption scandal implicating his youngest daughter Cristina, created the 'perfect storm' which pushed Juan Carlos to abdicate. Sayn-Wittgenstein was born a Danish national in 1964 in Frankfurt, Germany to parents Finn Bönning Larsen and Ingrid Sauer. She has been married twice, in 1989 to English businessman Philip Adkins, with whom he had a daughter, and in 2000 to Prince Casimir zu Sayn-Wittgenstein-Sayn, having ended her first marriage in 1995. She had a son with the prince in 2000, but they divorced in 2005, though she carried on using her married name. Currently residing in Monaco, Sayn-Wittgenstein was asked by the prince of the microstate to help bring in business. She used to be the director of Apollonia Associates, a company that connects 'people, platforms and organisations to create sustainable value'. Final De Partida, by Ana Romero (right), sold out within 24 hours of being published .
Book claims Juan Carlos was romancing Corinna zu Sayn-Wittgenstein.
Final De Partida - or End Game - by Ana Romero, sold out within 24 hours.
Author said at the book's launch the pair were 'a couple pure and simple'