Spain's Infanta Cristina faces charges over allegations that her husband embezzled millions of euros of public money.
Her husband of 16 years, Inaki Urdangarin, is suspected of having overcharged local authorities for organising sporting and tourism events.
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The Duke of Palma de Mallorca, a former Olympic handball player, denies wrongdoing and has not been charged. His wife, the Duchess of Palma de Mallorca, has been summoned to appear in court on 27 April.
Emails have come to light suggesting that she knew about her husband's financial affairs, according to Spain's El Pais newspaper.
Inaki and his former business partner Diego Torres are believed to have siphoned off around €5m given by local governments to the Noos institute, of which Inaki was partner from 2004 to 2006, to organise sporting and tourism events.
It is alleged that some of the money ended up in companies controlled by the duke and in offshore bank accounts. The 45-year-old was suspended from royal engagements in December.
It is not clear on what charges the princess was called to testify by judge Jose Castro.