The State Prosecutor's office Tuesday confirmed the court had taken charge of a new investigation after Spanish online daily elDiario.es reported that anti-corruption prosecutors were probing the alleged use by the former king, ex-Queen Sofia and some family members of credit cards linked to foreign accounts not in their names.
Juan Carlos' whereabouts have never been confirmed.
Juan Carlos, who abdicated in 2014, is already under investigation over his alleged role in a deal under which a Spanish consortium won a €6.7bn (£5.9bn) contract to build a high-speed rail line in Saudi Arabia.
Reports emerged in March suggesting Juan Carlos had received a $100m (€88m) payment from Saudi Arabia’s King Abdullah in 2008, three years before the contract was awarded. The king was covered by constitutional immunity while on the throne, but lost the protection when he stepped down to make way for his son, King Felipe.
He abdicated after a string of damaging scandals that culminated in a controversial elephant-hunting trip to Botswana while Spain was devastated by the effects of the 2008 financial crisis.