Cristina, 48, known as the infanta, is linked to the business affairs of her husband Inaki Urdangarin, who is under investigation for alleged embezzlement of public funds. She is herself being probed over her fiscal affairs.
Despite this, "there is nothing to justify indicting the infanta, so there is no preferential treatment," the head of the state prosecution service, Eduardo Torres-Dulce, told reporters.
The judicial investigation into Urdangarin and a separate probe of the tax affairs of Cristina, the youngest of the king's two daughters, have thrown Juan Carlos's family into the biggest popularity crisis of his four-decade rule.
In the first case, a court is investigating allegations that Urdangarin and a former business partner embezzled €6 million ($8 million) in public funds via the Noos Institute, a charitable foundation that he chaired.
The judge leading the investigation, Jose Castro, named Cristina, 48, as a suspect in the case in April and ordered her to appear in court for questioning, but that summons was later cancelled after public prosecutors appealed.
It was the first time a direct relation of the king had been called to appear in a court of law on suspicion of wrongdoing.
Castro then launched an investigation into whether Cristina had engaged in tax evasion or money-laundering.
A prosecutor in Spain's Balearic Islands, where the investigations are being carried out, said last week that he was against indicting Cristina in that case.
"There is not a single piece of evidence that could link Cristina de Borbon with criminal activities," Pedro Horrach, the Balearics chief anti-corruption prosecutor, wrote in a report.
A judicial source said that the court had on Monday received a report from the tax authorities on Cristina and was waiting to receive further information.
Cristina was a member of the board of Noos and with her husband jointly owned another company, Aizoon, which investigators suspect served as a front for laundering the embezzled funds.
Centre-right newspaper El Mundo said on Tuesday that Urdangarin's defence was proposing a plea bargain to avoid going to trial, asking for a sentence of less than two years.
Under Spanish law, a convict does not serve time in jail if the sentence is less than two years.
Torres-Dulce denied that report.
The scandal, along with the king's health problems and discontent over the recent recession, has raised debate about his future.
The 75-year-old king is due to undergo hip surgery on Thursday, his ninth operation in just over three years.