Princess Cristina, the sister of Spain's King Felipe VI, will be the first member of the royal family ever to appear in the dock after a judge Monday ordered her to be tried for tax fraud.
The historic decision comes after four years of investigations that plunged the royal family into crisis and contributed to the abdication of King Juan Carlos in June.
A court on the island of Majorca ordered Cristina, 49, to stand trial on two counts of being an accessory to tax fraud in connection with her husband's business affairs, in a written ruling seen by AFP.
It ordered her to pay a court bond of 2.7 million euros ($3.3 million) while a date is set for the trial. Cristina had already paid a deposit of 587,000 euros to cover her possible liability in the case.
She is accused of taking part in tax evasion by her husband, the former Olympic handball player Inaki Urdangarin. He is accused of embezzling and laundering millions of euros in public funds.
Cristina's lawyers say she is innocent of any wrongdoing.
Asked to comment on the ruling, a royal spokesman said it "has the utmost respect for the independence of the judiciary".
The trial is one of countless fraud and corruption scandals that have outraged Spaniards over recent years, including several cases of alleged financial misdoings involving the ruling conservative party.
Cristina Federica of Bourbon and Greece is the youngest daughter of Juan Carlos and sixth in line to the Spanish throne. She married Urdangarin in 1997 in a glittering ceremony in Barcelona.
The case is a big headache for Felipe who took the throne on June 19 promising an "honest and transparent monarchy".
Public prosecutors had called on the court to shelve the case, saying there was not enough evidence against Cristina and hinting that investigators were out to get the princess.
But investigating magistrate Jose Castro at the court in Palma de Majorca upheld a lawsuit brought by Manos Limpias (Clean Hands), a pressure group that has brought numerous corruption cases against public figures.
"We have made history," said the group's leader Miguel Bernad. "If it was not for our suit, there would have been no charges. Everyone is equal before the law."
- Public anger -
As well as Cristina and Urdangarin, the court on Monday ordered 15 other suspects to stand trial.
The court upheld a list of charges against Urdangarin including embezzlement and ordered him to pay a bond of 15 million euros.
Urdangarin is accused along with a former business partner of creaming off six million euros ($8 million) in public funds from contracts awarded to Noos, a charitable foundation.
Cristina sat on the board of Noos and Urdangarin was its chairman.
Investigators suspect that a separate company jointly owned by the couple, Aizoon, served as a front for laundering embezzled money.
Questioned in court by Castro in February, Cristina said she had simply trusted her husband and had no knowledge of his business affairs.
Castro grilled Cristina over accounts that indicated Aizoon money was used for personal expenses, including work on the couple's Barcelona mansion, dance lessons and Harry Potter books.
Judges last month dropped money-laundering charges against Cristina but upheld the lesser tax fraud charges.
The so-called Noos affair fanned public anger against the monarchy and the ruling class during the recent years of economic hardship in Spain.
The scandal soured the reign of Felipe's father Juan Carlos, who gave up the throne after 39 years so his son could freshen up the image of the monarchy.
Urdangarin and Cristina have been excluded from royal activities since 2011 when he was first named as a suspect.
Monday's ruling cannot be appealed and the accused have three weeks to present written court pleas in their defence, Castro ruled.
Cristina's lawyer Miquel Roca told reporters afterwards the princess was "surprised and upset" by the decision. Contrary to what the court ruled, he argued that there could still be grounds for her to appeal against it.