Spain's Princess Cristina (R) and husband Inaki Urdangarin leave court after appearing on charges of tax fraud in Palma de Mallorca, Spain, January 11, 2016. [Photo/Agencies]

PALMA DE MALLORCA, Spain - Spain's Princess Cristina appeared in court on Monday on charges of tax fraud, the first member of the royal family to stand in the dock, as a lengthy investigation into her husband's business affairs finally went to trial.

King Felipe's 50-year-old sister is one of 18 people on trial after a six-year investigation into the Noos Foundation, a charity run by her husband, Inaki Urdangarin, that prosecutors say was used to embezzle millions of euros in public funds.

Details of the investigation into the royal family emerged during the economic crisis when Spain was grappling with record unemployment and austerity and tapped into popular disgust at cases of high-level corruption among bankers and politicians.

Cristina arrived with her husband at the Mallorcan court to be greeted by a barrage of TV and press cameras.

Dressed simply in a grey trouser suit, she sat motionless in the court room fronted by a framed photo of King Felipe while the accusations were read out.

Her lawyers asked judges to drop the criminal charges against her as state prosecutors had previously said there was insufficient evidence to back up the accusations.

The charges were filed by the 'Clean Hands' anti-corruption organization using a Spanish legal instrument known as the 'people's accusation'.

The Spanish legal system is under pressure to get tough on corruption after a string of graft cases involving the establishment from the main political parties to bankers like former International Monetary Fund (IMF) head Rodrigo Rato.

"The law must be equal for all," said newspaper El Pais on Monday in an editorial which said the fact the probe had come to trial showed the legal system worked, albeit at a slow pace.