Minister Jose Manuel Garcia Margallo said the summoning of Princess Cristina affected the entire country's image and urged that the case be resolved rapidly.
"Anything that affects an institution that has been so important in Spain's transition (from dictatorship to democracy) and which is so important for Spain's image abroad, causes enormous concern," Margallo told reporters.
The subpoena announcement caused a sensation Wednesday and has been the main news item for all Spanish media.
The summons is a first for a member of the king's immediate family. The palace expressed surprise at the decision and welcomed a prosecutor's move to appeal it.
The 47-year-old princess, the youngest daughter of King Juan Carlos, has not been charged but must appear for questioning by Palma de Mallorca Judge Jose Castro on April 27. The investigation centers on whether her husband Inaki Urdangarin and his former business partner funneled about 5 million euros ($6.4 million) in public funds via the nonprofit Noos Institute they ran into private businesses they controlled.
"It's imperative that the judiciary should get to the bottom of the Noos case, not just because it's a serious case of corruption but rather because it's a scandal that is causing considerable damage to the prestige of the monarchy," the leading newspaper El Pais wrote Thursday in an editorial.
The royal family's troubles and corruption scandals affecting the country's two main political parties have greatly irked Spaniards, who are suffering through an economic crisis that has sent unemployment soaring to 26 percent.
Previously both the judge and the prosecutor had agreed there was not enough cause to call the princess in for questioning but the magistrate said new information had changed his mind.
The judge said while there was no indication that the princess took an active part in her husband's businesses, she was a board member on two of his companies and there was evidence she knew that Urdangarin used her name and status in his dealings, which benefited both of them. Castro said such evidence could lead the princess to be classified as an accomplice.
Urdangarin, 45, has already been questioned twice by Castro since the probe began two years ago. The royal family last year sidelined him from all official duties.
The princess went to work as normal Thursday at the Caixa bank foundation in Barcelona but did not comment on the case. She is seventh in the line of succession.