The payment marks Juan Carlos I's second attempt to regularise past undeclared income.
Former Spanish King Juan Carlos I has paid close to €4.4 million to the country's tax authorities in his latest bid to settle past undeclared income.
The 83-year-old former monarch already filed for settlement for another tax debt in December for a total amount of more than €678,000.
Juan Carlos' lawyer, Javier Sanchez-Junco, said in a statement Friday that the latest tax debt relates to the payments that a private foundation made on behalf of the former king for "several travel expenses and other services".
Zagatka, a Liechtenstein-based foundation, is owned by Alvaro de Orleans, a businessman and distant cousin of Juan Carlos. He has publicly admitted having funded some of the former monarch's private expenses. Payments-in-kind are subject to taxes under Spanish law.
Leading Spanish newspaper El Pais, which broke the story, and online news website El Espanol said that the foundation paid for flights with a private jet company for more than a decade up to 2018.
Investigations for financial wrongdoing
The payment was voluntary and tax authority had not asked for it, the lawyer's statement said. The former monarch's tax obligations "have been regularized," it added.
Under Spanish law, confessing to undeclared income and paying outstanding taxes allows offenders to avoid being charged with a crime.
The former king is the target of official investigations in Spain and Switzerland for possible financial wrongdoing.
One of them involves possible payments over a high-speed railway contract in Saudi Arabia. Those investigations prompted him to leave Spain in August for the United Arab Emirates. He has been photographed there in a luxurious hotel where Spanish media say he is living as a guest of Prince Mohammed Bin Zayed.
The Royal Palace declined to comment on the new developments.
PM appalled by 'uncivil behaviour'
"Regarding uncivil behaviour, I feel the same rejection as the majority of Spanish citizens,” Spanish Prime Minister Pedro Sanchez told reporters on Friday while praising the current king for his "exemplarity and transparency".
King Felipe VI, who rose to the Spanish throne after Juan Carlos' abdication in 2014, has distanced himself from his father.
Juan Carlos' scandals have deepened division in the Socialist-led government coalition, with junior partner United We Can making new calls for legal changes to improve the state's oversight of the crown.
Asked about the new development on Friday, Spanish Deputy Prime Minister Carmen Calvo said that the media reports were "an example that the country works and holds everyone accountable.''
"No one is allowed to break the law," Calvo told Canal Sur radio. "This comment is even more relevant today.''