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Spain's King Juan Carlos is to abdicate after almost four decades and his son, 46-year-old Crown Prince Felipe, will take over the throne, the country's prime minister announced Monday.

A constitutional amendment will be required to allow the change to take place, Mariano Rajoy said in a televised statement.

"His majesty, King Juan Carlos, has just communicated to me his will to give up the throne," Rajoy announced. "I'm convinced this is the best moment for change."

Spain's King Juan Carlos signs a letter confirming his abdication at the Zarzuela Palace in Madrid on Monday.Spanish Royal House via Reuters

A source told Reuters: "It's a political decision. He is abdicating given the new challenges in Spain because he thinks it's necessary to make way for the new generation."

Juan Carlos, 76, oversaw his country's transition from dictatorship to democracy but has had repeated health problems in recent years.

His popularity also dipped following royal scandals, including an elephant-shooting trip he took in the middle of Spain's financial crisis during which he broke his right hip and had to be flown from Botswana back to Spain for medical treatment aboard a private jet.

Spain's Crown Prince Felipe, left, and his father, King Juan Carlos, watch a military parade outside the parliament in Madrid on December 27, 2011. Spain's Prime Minister Mariano Rajoy said on Monday that King Juan Carlos will abdicate and Felipe will take the throne.JUAN MEDINA / Reuters, file

His image was also tarnished by the investigation of his son-in law, who is suspected of embezzling large amounts in public contracts. The king came to power in 1975, two days after the death of longtime dictator Francisco Franco.

The Associated Press and Reuters contributed to this report.

Alastair Jamieson