Spain's Juan Carlos I is to abdicate and be replaced by his son Crown Prince Felipe, sparking calls across the country for a referendum on the future of the monarchy.
Felipe, 46, is set to take over the Spanish throne from his 76-year-old father, following a new constitutional amendment to allow the surprise abdication, according to Spain's Prime Minister Mariano Rajoy.
But protests calling for a vote on whether the monarchy should continue have been called by republicans and left-wing groups for 8pm this evening, and the transition may not be as smooth as the Royals would hope.
"His Majesty King Juan Carlos has just informed me of his desire to renounce the throne and begin the process of succession," Rajoy said in a brief statement, calling the rulers a "tireless defender of our interests".
"I'm convinced this is the best moment for change," Rajoy added.
No reasons were given, with Rajoy saying the monarch would expand on his decision. Although not Europe's most elderly monarch (that would be Queen Elizabeth II), he has suffered with bouts of ill health in recent years, undergoing five hip operations.
Speaking from Madrid, Vanesa Rodríguez, editor-in-chief of HuffPost Spain, said the announcement had taken the media by surprise. "We were expecting the announcement of a government reshuffle," she said. "Although the abdication was a possibility that the press had been analysing for some time, the king had never publicly acknowledged that it was an option in the immediate future.
"But Juan Carlos, whose health is weakening, has long contemplated yielding the throne to Prince Felipe and he has now acknowledged that he had decided to abdicate in January, when he turned 76-years-old."
There is "large public disaffection" with the monarchy, said Rodríguez, with a survey showing people gave it 3.96 out of 10.
"Most political parties on the left are in favour of opening a process of change now and they want to be able to ask citizens whether to continue with the monarchy or opt for a change," she added.
Juan Carlos came to power in 1975, two days after the death of Spanish dictator Francisco Franco. He has ruled for more than 38 years, but has seen his once immense popularity and that of the Royal Family dwindle in recent years following a number of embarrassments.
He has endured a long-running corruption probe into his younger daughter, the Infanta Christina, and son-in-law Inaki Urdangarin, suspected of fraudulently obtaining millions in public funds.
Once seen as the king with the common touch, that reputation disintegrated after he caused widespread furore in Spain by going on an lavish secret elephant-hunting trip in 2012, at the height of the Spanish financial crisis.
"An impression of remote privilege was fuelled by spectacular gaffs," Mary Vincent, Professor of Modern European History at the University of Sheffield told HuffPost UK.
“Yet, during his long reign, opinions of the king varied considerably. When he came to the throne in November 1975, the leader of the (illegal) Communist opposition, announced that he would go down in history as ‘Juan Carlos el breve’ (Juan Carlos the brief).
“Undoubtedly, Juan Carlos’s finest hour came during the attempted coup of 21 February 1981 when, with parliament being held hostage by armed civil guards, and tanks in the streets of Valencia, the king addressed the nation on television in the early hours of the morning, and ordered the troops back to barracks.
"When, the following weekend, three million Spaniards took to the streets in support of democracy, one former Francoist minister remarked that even the communist clenched-fist salute as acceptable ‘if done while shouting Long Live the King’.
"He was instrumental in the transition to democratic government and became Spain’s first genuine constitutional monarch. It seems fitting that today’s announcement emphasises institutional continuity and parliamentary procedure.”
Juan Carlos has one elder daughter, the Infanta Elena, and Felipe is the youngest of the three children with wife Queen Sofia, a former princess of Greece and Denmark.
The Prince's approval ratings are high, at 66%, according to Reuters, compared to those of his father who has just 42% of people believe he is doing a good or very good job. Felipe is married to former journalist Leticia Ortiz and the couple have two daughters.
David Cameron told reporters today that Juan Carlos has been "a great friend of the United Kingdom" and did "so much during his reign to help Spain's successful transition to democracy".
In April last year, Queen Beatrix of the Netherlands abdicated, handing over to her eldest son, Willem-Alexander, the first King of the Netherlands in more than 120 years. Close in age to Juan Carlos, the 75-year-old former monarch now has the title 'Princess'.
She retired after 33 years in the role, and originally became queen herself after the abdication of her mother.