MADRID (AFP) - A Spanish judge says he will wrap up a corruption probe into King Felipe VI's sister Princess Cristina and her husband Inaki Urdangarin on Wednesday, opening up the prospect of a historic criminal trial.

Less than a week after 46-year-old Felipe VI took the crown from his father Juan Carlos promising an "honest and transparent" monarchy, the judge's decision threatens to put damaging royal corruption allegations back in the headlines.

The investigating judge in Palma de Mallorca, Jose Castro, will issue the results of his probe on Wednesday, the court said in a brief statement sent to reporters by text message.

Under the Spanish legal system, the judge will recommend any suspects to be charged, details of the charges they may face and the names of those he may want to be sent for trial. A final decision, however, is taken by the Palma de Mallorca provincial court after considering any appeals.

Anti-corruption prosecutors have previously opposed the judge's decision to name Cristina, 49, as a criminal suspect for complicity in the allegedly dodgy business dealings of her husband, a former Olympic handball player.

Long thought of as untouchable, the smiling, blonde-haired Cristina, would be the first direct relation of the Spanish monarch in history to stand in the dock as a criminal defendant.

The judge has spent more than two years probing allegations that Urdangarin, 46, and a former business partner creamed off money from government contracts awarded to Noos, a charitable foundation he chaired.

He suspects them of embezzling six million euros ($8 million) in public funds altogether. Cristina was a member of the foundation's board and with her husband jointly owned another company, Aizoon, which investigators suspect served as a front for laundering embezzled money.