The son-in-law of Spain's King Juan Carlos has been summoned to appear before a judge as a suspect in a corruption case.

The case surrounding Inaki Urdangarin, husband of the king's daughter Cristina, has been front-page news for weeks and a public relations nightmare for the royal family, coming at a time of acute economic crisis in Spain.

But the situation worsened when Judge Jose Castro on the Mediterranean island of Mallorca named Urdangarin as a formal suspect in a criminal probe. The Balearic Islands Superior Court of Justice said Urdangarin is to testify on February 6 in Palma, the capital.

Urdangarin is suspected of siphoning money from public contracts awarded from 2004 to 2006 to a non-profit foundation he headed.

An official at the Royal Palace declined comment other than to say it "respects the decisions of judges".

Spain has nearly 22% unemployment, a stagnant economy, and mountains of debt, so alleged shady business dealings by a member of the royal family look terrible for the Spanish monarchy.

On December 12 the Royal Palace shocked the country by announcing Urdangarin would for the time being stop taking part in official ceremonies involving the royal family.

And in an unprecedented show of transparency, the palace this week made public the details of the stipend the royal family receives from the national budget.

It said, for instance, that King Juan Carlos earns 292,552 euro (£244,600) a year in salary and expenses and his son, Crown Prince Felipe, roughly half that amount.

Spanish newspapers have quoted investigators as saying Urdangarin is suspected, among other things, of having taken some of about six million euro (£5 million) his non-profit foundation received from the regional governments in Valencia and the Balearic Islands for organising events such as sports seminars and diverting it to for-profit companies Urdangarin ran.