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The once-popular, now scandal-ridden, king left Spain before Monday's announcement, local media said, with no indication of where he may have gone.
The bombshell move by the 82-year-old King Emeritus has stunned Spaniards and left them divided over whether the 82-year-old, who keeps the title of King Emeritus, was right to depart or should have stayed to face justice.
Pressure had been building for weeks on the former king and his son, King Felipe, to take action to shore up the monarchy, after Spanish and Swiss prosecutors started looking into allegations of bribes over a high-speed rail contract.
The palace statement quoted Juan Carlos' letter to Felipe as saying that he wanted to enable him to rule untroubled "amid the public repercussions that certain past events in my private life are generating".
"Guided by my desire to do what is best to serve the Spanish people, its institutions and you as king, I am informing you of my ... decision to leave Spain at this time."
King Felipe thanked Juan Carlos for his decision, underlining "the historic importance that his father's reign represents" for democracy in Spain.
Juan Carlos came to the throne in 1975 after the death of General Francisco Franco and was widely respected for his role in helping guide Spain from dictatorship to democracy.
But his popularity sank in later years due to a series of scandals, prompting him to step down in 2014.
Juan Carlos' lawyer, Javier Sanchez-Junco said in a brief statement that despite his decision to leave, the former king would "remain at the disposal of the prosecutors' office".
Amid much speculation over where he could be, Portuguese TV channel TVI24 and the Correio da Manha tabloid said Juan Carlos was in Cascais, a resort area near Lisbon, where he spent part of his childhood. They did not cite any sources.
Switzerland's La Tribune de Geneve newspaper reported in March this year that Juan Carlos had received $US100 million (NZ$150 million) from the late Saudi King over a high-speed rail contract.
This was followed by a regular drip-feed of allegations in the Spanish media, including that he would have given much of the money to a former mistress.
Through Sanchez-Junco, Juan Carlos has repeatedly declined to comment on the corruption allegations. Spanish monarchs have immunity during their reign but Juan Carlos' abdication potentially leaves himself vulnerable to prosecution.
Juan Carlos' departure comes at a difficult time for Spain, hit by one of Europe's worst coronavirus outbreaks at a moment when local politics is tense and polarised.
"Juan Carlos de Borbon's flight abroad is an act unworthy of a former head of state and it leaves the monarchy in a very compromised position," Iglesias said on his Facebook page.
His party's parliamentary group went further, saying in a statement there was no reason to continue with a monarchy "lacking the minimum ethical values," potentially opening a rift with its government partner, the Socialist Party, which defends the monarchy.
'PAY FOR IT'
Some voters also said he should not have left.
"If he has done it, he should pay for it and stay," Juan Casado, a teacher from Cordoba, said on a visit to Madrid, referring to the corruption allegations.
Others in Madrid regretted his departure, pointing to his role in Spain's return to democracy.
"This person has done a lot for Spain, with dark and bright moments, of course, but he has done more good than harm," pensioner Pilar Romero said.
Opinion polls have shown that Spaniards are roughly equally split over whether their country should remain a monarchy or become a republic - a step that is impossible under the current constitution, itself very hard to change amid a highly fragmented parliament.
Local media including El Mundo and broadcaster TVE said Juan Carlos had left Spain, with some, including La Vanguardia, saying he had already left on Sunday.
The Royal House and government officials declined to comment and Juan Carlos' lawyer did not respond to a request for comment.
The government had repeatedly asked Felipe to take steps to distance himself further from his father after he put an end to his father's palace allowance and renounced his own inheritance in March, following allegations of secret offshore accounts reported to be linked to the Saudi rail contract.
LIFE OF JUAN CARLOS
Born in Rome in 1938 to exiled pretender to the Spanish throne Don Juan and Maria de las Mercedes de Borbon y Orleans, Juan Carlos was caught up in a power game as Spanish dictator General Francisco Franco proclaimed support for the monarchy while striving to keep a grip on power.
Franco separated Juan Carlos from his parents when he was 10 and groomed the boy to be his successor. He was crowned two days after Franco's death in 1975.
ABOUT-TURN TO DEMOCRACY
To the dismay of the old administration which hoped he would continue Franco's legacy, the king quickly implemented reforms that led to democratic elections in 1977.
After an attempted military coup on February 23, 1981, Juan Carlos gave a television broadcast in support of the government and Spain's young democracy. His backing was key to preventing a return to dictatorship.
He married Greek princess Sofia in 1962 and had two daughters and a son. A tall, handsome man in his youth, he had a reputation as a womaniser. Excelling at sailing and skiing, he was also a keen big-game hunter.
In 2012, at the height of a recession that saw many Spaniards lose their jobs amid huge public spending cuts, details emerged of a lavish elephant-hunting trip Juan Carlos took to Botswana. He later apologised.
FAMILY TAX FRAUD
Corruption scandals circling the royal family closed in when his daughter, Princess Cristina, was accused of tax fraud in 2014 and became the first Spanish royal to stand trial. She was later acquitted, but her husband was sentenced.
WITHDRAWAL FROM PUBLIC LIFE
He abdicated in favour of his son, Prince Felipe, in 2014, and last year, Juan Carlos announced his decision to withdraw from public life, ending his remaining institutional functions and appearances from June 2019. In August last year, he successfully underwent heart surgery in Madrid.
SAUDI RAIL CONTRACT
In June 2020, Spain's supreme court prosecutor opened an investigation into Juan Carlos' involvement in a high-speed rail contract in Saudi Arabia that was granted to a group of Spanish companies in 2011.
STRIPPED OF ALLOWANCE
King Felipe renounced his own inheritance and stripped his father of his palace allowance in March after reports the latter received $US100 million from the late Saudi king and gave millions to a businesswoman called Corinna zu Sayn-Wittgenstein.