The former head of state has been under a cloud since various media reported that he allegedly received funds from Saudi Arabia and probes are now under way in both Switzerland and Spain.
Spain's Supreme Court announced in June an investigation to determine the legal responsibility of the ex-monarch — but only for acts committed after his abdication in 2014, because of the immunity he holds.
The suspicions centre on $100 million (85 million euros) allegedly paid secretly into a Swiss bank account in 2008.
After a series of media revelations, Socialist Prime Minister Pedro Sanchez spoke out in July saying "the entire Spanish population are receiving disturbing information which is troubling for us all, including me".
Juan Carlos ascended the throne in 1975 on the death of the fascist dictator Francisco Franco and ruled for 38 years before abdicating in favour of his son Felipe VI in June 2014.
He was a popular figure for decades, playing a key role in the democratic transition from the Franco dictatorship which ruled Spain from 1939-1975.
An inquiry opened in Spain in September 2018 following the publication of records attributed to German businesswoman Corinna Larsen, a former mistress of Juan Carlos.
She claimed he had received a commission when a consortium of Spanish companies were awarded a high-speed railway contract to link the Muslim holy cities of Mecca and Medina in Saudi Arabia.
Larsen told Swiss investigators he had transferred to her nearly 65 million euros in the Bahamas, "not to get rid of the money", but "out of gratitude and out of love", according to El Pais daily.