King emeritus Juan Carlos admitted this week that he had used funds not declared to the tax office for personal expenses.
The former monarch has made an extra declaration to the tax authorities covering a debt of 678,393 euros, "including interest and charges", according to his lawyer.
The office of Juan Carlos's son, King Felipe, did not comment, however the news of the voluntary back-tax payment re-emphasised the decision of the present king to cut ties with his father last March and give up rights to his inheritance.
The back tax refers to the use of under-the-radar funds to cover credit card use by some of the Royal Family for at least three years: 2017, 2018 and 2019. Neither King Felipe nor Queen Letizia are said to have benefited. Sources said that the amount being declared could be as high as 1.5 million euros.
King Juan Carlos has been living in Abu Dhabi since August. Courts are also investigating two other alleged offences, which his lawyers have denied; the transfer of 65 million euros to his former lover, Corinna Larsen in 2012 as "a show of gratitude" and 10 million euros found in offshore Jersey and supposedly linked to the former head of state.