The probe is latest of a string of legal inquiries into the finances of the scandal-hit 82-year-old who fled into self imposed exile in the United Arab Emirates in August.
His departure came as investigators in Spain and Switzerland were looking his financial affairs following revelations by his former mistress, German businesswoman Corinna Larsen.
Anti-corruption prosecutors opened their investigation at the end of 2019 but it only came to light on Tuesday with the publication of a story by online news site elDiario.es
Legal sources said they were looking into the origin of funds deposited in several Spanish bank accounts held by a Mexican business and an official in the Spanish Air Force, and whether the money in them had been used by the
Prosecutors had sent various legal requests abroad to determine whether the monies deposited in the accounts had been hidden from the Spanish tax authorities, the source said, without specifying which countries were involved.
If proven, the allegations could constitute a money laundering offence for which he could be prosecuted given that the movement of funds and use of the credit cards occurred after his abdication in June 2014, meaning he no longer had immunity as head of state.
The Supreme Court confirmed that anti-corruption prosecutors had registered a preliminary investigation into Juan Carlos but did not give any other details.
Contacted by AFP, the Sanchez-Junco law firm representing Juan Carlos made no comment on the investigation.
In 2018, Spanish prosecutors opened a probe into the Saudi high-speed rail contract, which was awarded to a consortium of Spanish companies, seeking to establish whether the king emeritus was paid a commission.
In March, Swiss daily La Tribune said Saudi's late king Abdullah had in 2008 deposited $100 million into a Swiss private bank to which Juan Carlos had access, prompting suspicions it was a kickback for the contract which was
awarded three years later.
On August 3rd, the octagenarian announced he was going into exile abroad to prevent his personal affairs from undermining the reign of his son King Felipe VI, prompting anti-monarchists to accuse him of trying to “flee justice”.
In a statement issued through his lawyer, in August, Juan Carlos denied trying to escape justice and said he would remain available to prosecutors.