Javier Sánchez-Junco, lawyer to the ex-monarch, said the declaration and payment to “the competent tax authorities” of €678,393.72 had been at Juan Carlos’s own initiative and had cleared his tax debt, including interest and surcharges.
“His Majesty King Juan Carlos remains, as ever, at the disposal of the public prosecutor’s office for any procedure or action it deems appropriate,” Mr Sánchez-Junco added.
Wednesday’s announcement comes after a year in which controversy has grown around the former king, who abdicated in 2014, even as the monarchy has been sucked deeper into the country’s highly polarised politics.
Corinna zu Sayn-Wittgenstein, a former lover of the ex-king, in March said that he had given her €65m in 2012 — money that originated from a $100m gift to Juan Carlos from the late King Abdullah of Saudi Arabia.
Soon afterwards, King Felipe VI, Spain’s reigning monarch and Juan Carlos’s son, renounced any legacy from the former king and took his father off the royal household’s payroll.
Legal authorities disclosed in July that prosecutors at the country’s supreme court were investigating Juan Carlos over suspected payments concerning a €7bn high-speed train project between the holy cities of Mecca and Medina that was awarded to a Spanish consortium in 2011.
In August, Juan Carlos installed himself in the seven-star Emirates Palace Hotel in Abu Dhabi after announcing that “guided by the conviction of providing the best service” to the country’s institutions and people, he had decided “to move, at this time, outside of Spain”.
After news reports that Juan Carlos had used credit cards paid by a third party, the state prosecutor’s office revealed in November the existence of a separate investigation into the former king and said it had alerted the country’s tax inspectorate and Data Protection Commissioner of a possible leak.
If such an investigation involved events after the ex-king’s 2014 abdication, it could be of greater legal significance than the inquiry into the 2011 rail contract, because as reigning monarch Juan Carlos enjoyed constitutional protection from prosecution.
In theory, Juan Carlos could be tried in the supreme court for actions after 2014. But, under Spanish law, someone cannot be prosecuted for tax offences if they settle an outstanding debt — as Juan Carlos’s lawyer says he now has — before formal charges are brought.
The Spanish media has reported in recent days that the ex-king has told friends of his desire to return to the country for Christmas. But Juan Carlos, long-feted for his role in steering Spain towards democracy in the 1970s and for standing up against a coup attempt in 1981, is now a contentious figure.
Even before Wednesday’s announcement, Pablo Echenique, the parliamentary spokesman for Podemos, the junior partner in Spain’s ruling coalition, said Juan Carlos’s tax declaration amounted to a “confession” that he had “defrauded the budget ministry”.
Government ministers have been more discreet. “You can perfectly well imagine [what I think] if I tell you that we are equal before the law and, particularly, those of us who have public responsibilities,” María Jesús Montero, budget minister, said on Wednesday.
The role of the monarchy has become politically charged at a time when Spain’s leftwing government is kept in office by separatist Basque and Catalan parties — to the fury of the right.
Last month 73 retired army officers wrote a letter to Felipe VI attacking the “socialist-communist government” and pledging their loyalty to the king “in these difficult moments for the fatherland”.
On Wednesday night, Pedro Sánchez, Spain’s prime minister, said it was important to maintain the presumption of innocence of the former king. “Democracy is working, the media is reporting what is happening, the justice system is functioning, the tax system is working, without any kind of interference,” he added. “The monarchy is not in danger.”
The Royal Palace did not respond to a request for comment.