She is one of the richest women in Spain, owns a dozen castles whose walls are hung with works by Goya, Velázquez and Titian and is a distant relative of King James II, Winston Churchill and Diana, Princess of Wales. Now, however, the 18th Duchess of Alba is giving away her immense personal fortune in order to be free to marry a minor civil servant.

According to Guinness World Records, Maria del Rosario Cayetana Alfonsa Victoria Eugenia Francisca Fitz-James Stuart y de Silva, born in Madrid's Palacio de Lira, has more titles than any noble on earth, being a duchess seven times over, a countess 22 times and a marquesa 24. As head of the 539-year-old House of Alba, her privileges include not having to kneel before the pope and the right to ride on horseback into Seville cathedral.

But the children of the duchess, 85, have until now blocked her plans to marry Alfonso Díez, 24 years her junior. The duchess and Díez, a civil servant in the department of social security who also runs a PR business, have been close friends for a number of years.

Her six children who, as she likes to point out, are all divorced, were all borne from her first marriage to Pedro Luis Martínez de Irujo y Artazcoz, son of the Duke of Sotomayor, who died in 1972.

The duchess, who is rumoured to have undergone extensive cosmetic surgery, shocked the nation when in 1978 she remarried, this time to the former Jesuit priest and intellectual, Jesús Aguirre y Ortiz de Zárate. Aguirre, who died in 2001, was illegitimate, something scandalous even in 1970s Spain.

In 2008 it appeared that the proposed marriage to Díez had been called off when the House of Alba issued a statement saying that the relationship "was based on a long friendship and there are no plans to marry". The statement came after an alleged telephone call from King Juan Carlos discouraging the duchess from marrying Díez.

But whatever the king thinks it now appears the duchess is going ahead with the marriage, and the details have now emerged of how she plans to overcome her children's opposition: by giving them their inheritance in advance, even though Díez has signed a document renouncing any claim to her wealth. "Alfonso doesn't want anything. All he wants is me," she said earlier this year.

According to a report published in Spanish newspaper El País, her eldest son Carlos inherits the Liria Palace in Madrid and the Monterrey Palace in Salamanca, as well as overall control of the family fortune. Much of the patrimony is managed by a foundation and, in return for tax breaks, belongs by law to the nation and cannot be sold.

However, the duchess's personal wealth is estimated at between €600m and €3.5bn and she has been able to give her children and eight grandchildren a palace each, as well as a chunk of the thousands of acres of Spain that she owns. Her only daughter, Eugenia, inherits an estate in Ibiza and a further 600 acres near Seville.

The duchess insists she is not that wealthy. "I have a lot of artworks, but I can't eat them, can I?" she has protested. The art that she cannot eat includes, aside from hundreds of paintings, a first edition of Don Quixote, Columbus' first map of America and the last will and testament of Fernando the Catholic, father of Catherine of Aragon.