Madrid (CNN) -- The son-in-law of Spain's King Juan Carlos has been summoned to testify before a magistrate Saturday as a suspect in a fraud scandal that has created unprecedented problems for the popular royal family.
Inaki Urgangarin, who was granted the title of Duke of Palma when he married the king's youngest daughter, Princess Cristina, in 1997, is under investigation for allegedly diverting public funds that were earmarked for his foundation for private use.
Through his lawyer, Urdangarin has denied the charges.
No trial has been set in the case, which has riveted national attention.
"People can understand if politicians, businesses or unions do this, but when it happens in the royal family, that is a hard blow for all Spaniards," said Gerardo Correas, an expert on royal affairs who runs the International School of Protocol in Madrid.
The investigation, officially secret, has been top news for months in Spain as details were reported. The court in Palma de Mallorca has publicly confirmed only the basic charge of misappropriation of public funds and named Urdangarin and some former associates as suspects.
Urdangarin, an Olympic medalist in handball, led a private foundation that secured lucrative contracts from regional governments to promote sports and tourism.
As the case gained notoriety, the royal palace announced last December that the Duke of Palma would not take part in official ceremonies, which is a key role for members of the royal family.
The royal household is widely respected for the king's role -- as head of state -- in guiding Spain to democracy after the dictatorship of Francisco Franco.
Also last December, the royal family publicly revealed its finances for the first time in an effort to boost public confidence and transparency.
Then the king, in his annual Christmas address, issued a warning to public officials.
"Fortunately, we live in a state of law and any objectionable action should be tried and punished according to the law," the king said. "Justice is for all."
Correas said the king "was clearly referring to the Duke of Palma, distancing himself from the duke."
Madrid's wax museum already has moved the image of Urdangarin away from the royal family. He now stands alone in the section for sports figures.
Urdangarin will mount a vigorous defense, said his lawyer, Mario Pascual Vives.
"I am still convinced that he is innocent," Pascual Vives said. "And I have always said that her highness, Princess Cristina, is not involved in this at all."
Urdangarin and the princess have lived with their children in Washington, D.C., in recent years.
Speculation was high earlier Friday about whether he would have to walk past cameras Saturday morning to enter the courthouse -- as other suspects under investigation in the case have done -- or whether he would enter in the courthouse in a vehicle, largely out of sight.
The court late Friday issued a statement announcing that Urdangarin will be allowed to enter the courthouse in a vehicle on an "exceptional" basis made strictly for security reasons. Other suspects in the case have had to walk past cameras to enter the building.
The testimony will be in a closed-door session, with no cameras allowed, in the investigating magistrate's chambers. Prosecutors and Urdangarin's defense lawyer would also attend.
The judge has not said publicly when the investigation might conclude. After that, the judge may set a trial, and indict suspects who currently face only preliminary charges, or clear these suspects of all charges.