A judge questions Spanish King Juan Carlos's daughter Cristina on Saturday over fraud allegations in a corruption scandal -- a historic first for the troubled royal family.

The rare royal spectacle takes place at a court on the island of Majorca, where for decades the family sunbathed and sailed yachts in the summer.

The holiday island is now the centre of an embarrassing scandal that has turned much of the public against the royals and raised doubts over the future of the monarchy.

Ranks of television cameras and reporters will be waiting at the court, whose press office said 300 journalists were accredited to report on the closed-door hearing, scheduled for 10:00 am (0900 GMT).

Inside, red velvet chairs were lined up in the courtroom where Cristina was to sit before investigating judge Jose Castro, overlooked by a photograph portrait of her own father, Juan Carlos, 76.

Castro could not confirm whether Cristina, 48, would provide the unprecedented sight of a Spanish princess arriving on foot at court.

For security reasons the court has given her permission to drive right up to the door, which would save her a humiliating walk in front of the world media's lenses.

Pro-republican campaign groups have vowed to demonstrate nearby.

Long thought untouchable as a royal, Cristina now faces public scrutiny over accusations that she was complicit in the business dealings of her husband, who is also under investigation.

Neither Cristina nor her husband, former Olympic handball player Inaki Urdangarin, has yet been formally charged with any crime and both deny wrongdoing.

- Mounting public anger -

Castro has spent more than two years investigating allegations that Urdangarin and a former business partner embezzled six million euros ($8 million) in public funds via a charitable foundation.

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.

State prosecutors say there is no case to answer against Cristina but the judge has admitted suits brought by pressure groups.

It is the first time a member of the royal family has ever gone to court as a suspect. The hearing follows more than two years of mounting anger against the elite in a Spain battered by recession.

Juan Carlos won widespread respect for helping steer Spain to democracy after the death of the dictator Francisco Franco in 1975.

But the royals' popularity has plunged since the case against Urdangarin opened three years ago.

The king's woes were worsened by a luxury elephant-hunting trip he made to Africa in 2012 as his subjects suffered in a recession.

These scandals and the sight of the king looking frail on crutches in his rare public appearances have raised debate about the future of his reign.

A recent poll showed 62 percent of Spaniards in favour of his abdication. Support for the monarchy in general fell to just under half.

For years the family were photographed by the press spending their summer holidays on Majorca, where they stayed at the Marivent Palace, a red-roofed edifice overlooking the Mediterranean.

But they have kept a low profile since the scandal broke and Cristina was not expected to sleep there this weekend. She was due to arrive on the island by plane on Saturday morning, media reported.