Princess Cristina of Spain is due to be questioned in court in connection with a corruption scandal involving the business dealings of her husband Inaki Urdangarin.
It will be the first time in history that a member of Spain’s royal family has appeared in court as the subject of a criminal investigation.
Inaki Urdangarin is alleged to have defrauded regional governments of millions of euros of public money.
Infanta Cristina and Inaki Urdangarin deny any wrongdoing, and have not been charged.
Spain’s royal household admits the case has damaged the reputation and credibility of Spain’s royals, and, partly because of this scandal, the popularity of King Juan Carlos has fallen in recent years.
Pro-republican campaigners have vowed to demonstrate near the court.
Princess Cristina, 48, will be accompanied by her lawyer as she answers dozens of questions from a judge in a closed-door hearing.
Proceedings are set to start at 10:00 local time at the main court building in Palma de Mallorca, capital of the Balearic Islands.
The judge has named her as a fraud and money-laundering suspect.
The allegations relate to a supposedly not-for-profit organization called Noos, of which Inaki Urdangarin was president.
The foundation staged a series of sporting events for the regional governments of the Balearic Islands and Valencia.
Inaki Urdangarin is accused of organizing the events at hugely inflated prices.
With a former business partner, he is alleged to have received a total of 5.6 million euros ($7.5 million) in public money.
Princess Cristina is suspected of spending some of that money on personal expenses.
There are also questions about what Princess Cristina knew about the alleged wrongdoing of her husband.
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