MADRID (AFP) - Once almost taboo, Spaniards are now talking seriously about a possible abdication by 76-year-old King Juan Carlos as he battles a string of health woes and his youngest daughter fights a corruption scandal.

The monarch, widely respected for guiding Spain to democracy after General Francisco Franco's death in 1975, has plunged in the polls as royal scandals accumulate and his own health comes increasingly under question.

"There is no shortage of problems for the Spanish monarchy, they have reason to be worried," said Mr Antonio Torres del Moral, a law lecturer at Spain's Uned university.

"Surely the day that the King abdicates is close," Mr Torres told Agence France-Presse (AFP).

"The monarchy is going through its worst moment," added the legal expert, who as recently as last year argued that it was not the right time for the King to abdicate.

Since last year, King Juan Carlos's problems have grown.

A judge on Tuesday declared the King's youngest daughter, 48-year-old Cristina, a suspect in alleged tax and money-laundering crimes.

He summoned her to testify on March 8 at his court in Palma on the Mediterranean island of Majorca, a first for a direct relative of a Spanish King.

Cristina has been linked to the business affairs of her husband, former Olympic handball player Inaki Urdangarin, who has been under investigation for alleged embezzlement of public funds.

The princess' summons was issued just a day after King Juan Carlos appeared at a military parade while on crutches.

It was his first public appearance since undergoing surgery to replace his left hip on Nov 21, his ninth operation since May 2010.

But his performance was not reassuring.

The King stammered several times during his 10-minute speech, fuelling public concerns over his health.

"The latest hit to his image that he suffered during the military parade was tremendous because we saw someone whose health was delicate, who lacked energy, who had lost his strength," Mr Cesar de la Lama, the author of the first authorised biography of the King, told AFP.

"The next four, five, six months will be key to the path the monarchy takes. Either these problems are forgotten or by the summer we will have an abdication. It is a critical moment," added Mr De La Lama, who covered the King during his 40 years as a reporter for Spanish news agency EFE.

Mr Torres del Moral of the Uned university said the King should step aside if his health does not improve.

"If the deterioration in his health is not just a consequence of the operations he has had but is something more permanent, then the abdication should take place as soon as possible," he said.

The royal family's standing has been damaged not only by the financial corruption scandal, however.

King Juan Carlos himself outraged many Spaniards when it emerged he had taken a luxury African elephant-hunting safari in 2012 as his subjects suffered in a job-destroying recession.

The number of people with a high or very high opinion of the King fell nine percentage points over 2013 to 41 per cent, according to a poll published on Sunday in daily newspaper El Mundo.

The proportion wanting him to abdicate in favour of Prince Felipe, 45, surged by 17 percentage points to 62 per cent, according to the study by pollster Sigma Dos carried out in late December before the military parade and the judge's move to name Cristina as a suspect.

Mr Fermin J. Urbiola, a journalist who has written several books on the King, said the key role the monarch still plays has been overshadowed in recent years by Spain's economic crisis and the corruption scandal that has engulfed his daughter.

"I don't believe that in this context of an economic crisis, of scandals, the efforts made by the King, which exist and are enormous, are getting the attention of the Spanish people," he told AFP.

The royal palace and the King have firmly denied any thoughts of an abdication.

"I want to express to you, as King of Spain, my determination to continue the faithful fulfilment of the mandate and the powers attributed to me," the King said in his an annual Christmas Eve address.