The Infanta Cristina's petition will join a similar appeal which was lodged by public prosecutors on April 3rd, the source added.
Earlier this month Judge Jose Castro postponed the princess' corruption hearing, originally scheduled for April 27th, until after the prosecutor's appeal had been heard.
The graft case is centred on allegations of embezzlement and influence peddling against Cristina's husband, former Olympic handball player Iñaki Urdangarin, and his former business partner Diego Torres.
The pair are suspected of overbilling regional governments to stage sports and tourism events, and then syphoning off money to the non-profit Nóos Institute, which Urdangarin chaired from 2004 to 2006.
The Infanta — the seventh in line to the Spanish throne — had seemed set to avoid being snared by the case.
But Castro decided on April 3rd to name her as a suspect and ordered her to appear in court, citing new evidence — including e-mails provided by the court by her husband's former business partner — which he said raised doubts that she really was unaware of the business operations of Nóos.
The judge said that for the probe to be complete and to show that justice treats all equally she would have to be questioned.
The decision, the first time a direct relative of the king has been called to appear in a court of law on suspicion of wrongdoing, has created an unprecedented crisis for the Spanish monarchy, whose approval ratings have plunged due to a series of missteps.
King Juan Carlos was forced to apologize to the nation last year after he fractured his leg while on an elephant hunting trip to Botswana, at a time when Spain was mired in a steep recession.
Support for the monarchy in Spain has fallen to an all-time low of 53 percent, according to a poll published in a centre-right daily newspaper earlier this month.