Spain does not have a law that precisely regulates abdication and succession. Rajoy's cabinet was due to hold an extraordinary meeting on Tuesday to set out the steps for the crown prince to take over as King Felipe VI. The transition will probably be accomplished by passing a law through parliament, where the PP has an absolute majority.

Need for change

"We've been hearing continuously over the last few months about the necessity for deep change. The feeling is that the European elections have been a turning point and I believe the decision has been made in this context," said Rafael Rubio, constitutional expert at Madrid's Complutense University.

A survey in January by Sigma Dos found 62 percent of Spaniards in favour of the king stepping down, compared with just 45 percent a year earlier. Only 41 percent had a 'good' or 'very good' opinion of the king.

Felipe, by contrast, has a 66 percent approval rating and the poll indicated that most Spaniards believed the monarchy could recover its prestige if he ascended the throne.

"Felipe has a lot more energy to do the job," said 36-year-old student Alfonso Romero.

Political analysts speculated that Felipe may try to promote dialogue between Rajoy and Catalan President Artur Mas, who is leading a movement to break away from Spain. Mas said on Monday that Felipe's succession would not dissuade him from trying to hold a referendum on independence for Catalonia in November.

The prince, who has had a growing role in ceremonial events in the past year, is seen as more practical and in tune with current affairs than Juan Carlos, a jovial skier and sailor once beloved for his common touch and seen as much more accessible than the older generations of British royals.

Juan Carlos will be the third European monarch to abdicate in just over a year. Albert of Belgium left the throne to his son Philippe on July 2013 and Queen Beatrix of the Netherlands stepped down in April 2013 to make way for her son Willem-Alexander.

Felipe married divorced journalist Letizia Ortiz in 2004 and they have two daughters. The royal family began a Twitter feed on their 10th wedding anniversary, May 21, with tweets on both Juan Carlos and Felipe's weekend visit to El Salvador for the swearing in of President Salvador Sanchez Ceren.

The prince was in Spain on Monday but had no official events planned until Tuesday, when he is scheduled to appear with the king at the El Escorial monastery and former royal palace.