The blonde-haired, 48-year-old Cristina, once known for her easy smile, was summoned Tuesday to appear on March 8th before a court in Palma on the Mediterranean island of Majorca as a suspect in alleged tax and money-laundering crimes.
It will be the first time in modern history that a direct relative of the Spanish king has faced court as a suspect.
The judge in Palma on the island of Majorca, Jose Castro, has been investigating the corruption scandal involving Cristina's husband Iñaki Urdangarin, a former Olympic handball player, since 2010.
Castro had summoned Cristina on suspicion of corruption linked to her husband's activities once before, in spring 2013, but that decision was overruled following an appeal by the prosecutor.
The judge then opened a new line of inquiry, scrutinising her tax declarations and bank accounts for incriminating travel, restaurant or family party expenses.
Whether or not the court summons leads to formal charges, it represents a grave new blow to the prestige of the princess and to the institution of the Spanish monarchy.
Already, the image of the royals has suffered during the investigation into Urdangarin and his former partner Diego Torres, who are suspected of syphoning off money paid by regional governments to stage sports and tourism events to the non-profit Noos Institute, which Urdangarin chaired from 2004 to 2006.
Both men have denied any wrongdoing and neither has been formally charged with any crime.
Cristina has kept low profile
Cristina, who works as the director of social welfare programmes at Barcelona-based financial services group La Caixa's charitable foundation, has kept a low profile since the scandal broke.
La Caixa posted her to Geneva last year to coordinate its work with international agencies.
"The deterioration in Princess Cristina's image has no turning back, at least for a long time," Emilio de Diego, a history professor at Madrid's Complutense University, said before the latest summons.
"Princess Cristina has always been the wayward daughter of the family, I think some of the monarch's mistakes when it comes to family matters began there, by tolerating that she work at a private firm like La Caixa and collect a salary without renouncing her status as a princess," he added.
The corruption case has also ruined the public image of Urdangarin as the ideal son-in-law which he had enjoyed since he married Cristina in a lavish ceremony in Barcelona on October 4th, 1997.
A fan of various sports, especially sailing, Cristina — the seventh in line to the Spanish throne — met Urdangarin at the 1996 Olympics in Atlanta where he was competing with Spain's handball team, which won a bronze medal.
"She likes muscular men who are athletic, tall and sexy," said Andrew Morton, the author of a biography on Princess Diana who recently published a book on Cristina and three other female members of Spain's royal family.
"She is enormously competitive and obstinate," he added.
In 2009, Cristina and her husband, along with their four children, moved from Barcelona to Washington where Urdangarin took up a job as an executive director of the US subsidiary of Spanish telecoms giant Telefonica.
The couple, who were given the title of Duke and Duchess of Palma when they wed, were living in the United States when the allegations of corruption at the Nóos Institute first broke in Spain.
In August 2012, the couple and their four children returned to Barcelona, where they own a mansion in the exclusive Pedralbes area that reportedly cost around €6 million ($8.0 million).
Half of that mansion has since been placed under a court embargo, along with other properties, to cover a €6.1-million bond for Urdangarin's liability in the corruption case.
Spanish media reactions
Spain's press on hailed the news that King Juan Carlos' youngest daughter has been declared a financial fraud suspect as evidence that all are equal under the law.
"Far from endangering the pillars of the state or the future of the monarchy, the judge's decision is a good symptom of the democratic health of a society facing severe criticism of the functioning of its institutions," Spain's leading daily El País said in an editorial.
"There is nothing strange about a judge proceeding in a criminal case. The opposite would be untenable: not asking any question of the king's daughter because of who she is."
Right-leaning daily El Mundo said the latest turn in the case showed that Cristina should have agreed to appear when the judge first tried to summon her on suspicion of abusing her royal position for financial benefit.
"What this case has shown is the spectacular failure of a strategy, which, while trying to shield the monarchy, has left it scorned," it said.
Among those joining the strategy, El Mundo said, were the prosecutor, defence lawyers, the tax authorities, the royal household and the media, "which hypocritically applaud the king for defending justice that is 'equal for all' while at the same time showing support for burying activities of the princess that are worthy of judicial investigation".
Princess should not get special treatment
The pro-monarchy ABC, however, protested that the media had already started its own trial.
"The princess should not receive any special treatment, and ABC defends that. Like any Spaniard she will have to abide by what the judicial system finally decides," the paper said.
"But neither should she be sentenced ahead of time in a parallel trial by media, whose obvious motive is to seek an audience or to sell more papers on the back of the scandal, whether it is true or not."
ABC praised the palace for a statement of "respect for judicial decisions" that it issued after the court summons.
The head of the royal household, Rafael Spottorno, said in a television interview broadcast that the investigation had become a "martyrdom" for the royal family which has seen its popularity plummeting in the wake of recent scandals.