The Spanish king's sister, Princess Cristina, is to face a tax fraudtrial over alleged links to her husband's business dealings.
It is the first time for modern Spain to put a royal in the dock to face trial.
Her husband Inaki Urdangarin is accused of embezzling millions in public funds with a former business partner.
The couple deny wrongdoing. Prosecutors in Palma, Majorca, say Mr Urdangarin's sports foundation misused public money.
It is alleged that €5.6m (£4.6m; $7.5m) of public money went missing from the Noos Institute, a charitable sports foundation, when Mr Urdangarin was in charge of it.
Princess Cristina, 49, is under suspicion over €2.6m of that money. She is the sister of King Felipe VI, and youngest daughter of the former king, Juan Carlos.
The allegations relate to business affairs in 2007-2008. The affair has cast a long shadow over the Spanish royal family since the investigation was launched in 2010.
Mr Urdangarin and his then business partner, Diego Torres, allegedly used the Noos Institute to organise events for the regional governments of Valencia and the Balearic Islands at hugely inflated prices.
The princess is expected to sit in the dock with 16 other suspects. With her husband she co-owned a company called Aizoon.
A far-right trade union called Manos Limpias ("Clean Hands") brought a civil action over the affair and wants an eight-year prison sentence for the princess and 26.5 years for her husband.
Princess Cristina is sixth in line to the throne. Her wedding in 1997 to the then Spanish professional handball player Inaki Urdangarin was celebrated in a fairy-tale atmosphere.
But in recent years opposition to the monarchy has grown. Juan Carlos abdicated on 18 June, after months of poor health.
Juan Carlos was widely admired for the way he steered Spain's difficult transition to democracy after the death of the dictator Francisco Franco in 1975.
But before his abdication Juan Carlos was criticised over controversial hunting trips, at a time when the Noos scandal was regularly making headlines. Republican feelings have also grown in Catalonia's strong pro-independence movement.