The Palma de Mallorca court announced that the 47-year-old princess will be called in for questioning on April 27. She has not been charged with any offense and faces no restrictions on her freedom. Her husband, Inaki Urdangarin, a lead suspect in the investigation, has also not been charged with any crime.
The court summons is a first for a member of the king's immediate family. The Royal Palace expressed surprise at the judge's decision and would not comment about what the princess' defense will be.
The announcement comes after a year of health and image problems for 75-year-old King Juan Carlos, once one of Spain's most popular figures, widely admired for his role in helping steer Spain to democracy in the 1970s after decades of dictatorship.
But the Mallorca corruption case and other issues have eaten away at Spaniards' admiration for their royal family -- especially as a crippling economic crisis has widened the gap between rich and poor.
"This is the toughest blow the royal family has received within the last few years, or ever since I can remember," Jan Martinez Ahrens, deputy editor of El Pais, the country's leading newspaper, told The Associated Press.
Investigating authorities allege that Urdangarin and his former business partner Diego Torres funneled about 5 million euros ($6.4 million) in public funds to companies they controlled. The two ran a nonprofit organization called the Noos Institute, through which the funds were channeled and of which the princess was a board member.
In a court document Wednesday, investigating magistrate Judge Jose Castro said the princess was a board member on two of her husband's companies. The magistrate added there was evidence the princess was aware that her husband had used her name and status in his dealings, from which both had benefited. Castro said such evidence could lead the princess to be classified as an accomplice.
Urdangarin, 45, has already been questioned twice by Castro since the probe began two years ago.
The magistrate said for the investigation to be completed and to show that justice treats all equally, the princess must be questioned.
Speculation that the princess could be involved began when Torres reportedly began turning over to the judge copies of emails he had exchanged with Urdangarin.
The judge recently set joint bail of 8.2 million euros ($10.5 million) for Urdangarin and Torres and said the two "ignored the rules of public contracting as long as they achieved their aim, which was no other than to divert public funds for their own benefit or that of others."
He said the two decided to make as much as possible out of Urdangarin's relationship with the Royal Palace in their dealings with public and private entities.
The bail was not paid and the court has now begun a process to freeze their property and other assets.
Urdangarin is a former professional and Olympic handball medalist. The deals he landed were for things such as organizing seminars or using sports as a tourism lure.
Urdangarin comes from a wealthy Basque family but is not nobility; he became Duke of Palma because Cristina is Duchess of Palma. The couple has four children. Cristina, whose full name Cristina Federica de Borbon y Grecia, is the youngest of the king's two daughters and the middle child of his three children. She is seventh in the line of succession.
The royal family last year sidelined Urdangarin from all official royal activities. It has also removed him from the family website.
The king's fortunes first turned sour last year when it was revealed that he broke his hip while on an unannounced luxurious African safari to hunt elephants at a time when ordinary Spaniards were suffering under government austerity measures.
He has also had several other health problems and in recent months has looked decidedly frail. In March, the king was operated on for a herniated disc in his lower spine. The operation was the head of state's fourth in 12 months.
When Dutch Queen Beatrix, also 75, announced in January that she would abdicate and pass the crown to her eldest son, some said the same should happen in Spain. The palace, however, has said Juan Carlos has no plans to fast track the succession of Prince Felipe, his youngest children.
There is no major movement in Spain to eliminate the monarchy and restore a republican form of government. So far, only the leader of the regional Catalan Socialist Party has called openly for Juan Carlos to abdicate and allow his son to take the throne.
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