Spain's Princess Cristina, sister of King Felipe, has been found not guilty of charges of being accessory to tax fraud following a investigation into her husband's financial affairs.

Cristina's husband, Inaki Urdangarin, was sentenced to six years and three months in prison on charges of fraud and tax evasion, the authorities said in a statement on their website.

Cristina will pay a fine because she has civil responsibility, they said.

She was one of 18 defendants in the year-long trial that followed a probe into a charity run by her husband that prosecutors say was used to pilfer millions of euros in public funds.

The trial has been closely followed in a country jaded by high-level political and banking corruption cases and where levels of inequality have grown in the wake of the 2008 financial crisis.

Cristina was charged with two counts of being an accessory to tax fraud and, if found guilty, could have faced up to four years in prison for each charge. Any criminal indictment would have been unprecedented for a relative of the royal family.

As part of efforts to modernise the monarchy, King Felipe removed Cristina and her sister Elena from royal duties when he ascended the throne in 2014. The following year he stripped Cristina of her title of Duchess of Palma.

Cristina's husband, a former Olympic handball player, faced nine charges including fraud and tax evasion and carrying a combined potential jail sentence of 19-1/2 years.

Neither Cristina nor her husband were called to appear in court for the verdict, a court spokeswoman said. Both have lived in Switzerland with their four children since 2013 and have always denied any wrongdoing.

Prosecutors accused Urdangarin of using royal connections to win public contracts to put on events through his non-profit organisation, the Noos Foundation. He overcharged for the events and then hid the proceeds abroad, they said.

Some of the foundation's money was transferred to a private company used to pay for personal family items like holidays, home furnishings and theatre tickets, prosecutors said.

The investigation into Cristina and her husband served to heighten feelings among Spaniards that the royal family was out of touch with ordinary people.

Public support for the family was further eroded when it emerged former King Juan Carlos went on an elephant hunting expedition in Africa at the height of the financial crisis. He abdicated in favour of Felipe, his youngest son, in 2014.

The other 16 accused include the former head of the Balearic Islands government and other high-ranking officials from the islands and the Mediterranean coastal region of Valencia.