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King Juan Carlos of Spain is abdicating, Prime Minister Mariano Rajoy has announced.
Juan Carlos, who is 76, has ruled since 1975, taking over after the death of dictator Francisco Franco.
The king's son, Crown Prince Felipe, 45, will take over the throne.
For much of his reign, Juan Carlos was seen as one of the world's most popular monarchs, but recently many Spaniards have lost confidence in him.
His reputation has been tarnished by a long-running corruption investigation into his daughter and her husband.
Support for the king fell further when it was discovered he had been on a lavish elephant hunting trip to Botswana in April 2012, in the middle of Spain's financial crisis.
"His Majesty King Juan Carlos has just informed me of his desire to renounce the throne and begin the process of succession," Mr Rajoy said in a short statement.
He gave no reason for the decision, saying the monarch himself would explain, but Juan Carlos' health is failing and he has had a number of hip operations in recent years.
Spain does not have a precise law regulating abdication and royal succession, and Mr Rajoy said ministers would hold a special meeting to discuss the process by which the prince would take over as Felipe VI.
The prime minister said Juan Carlos had been a "tireless defender of our interests".
"I'm convinced this is the best moment for change," he added.
