MADRID  — Almost two thirds of Spaniards want their king to abdicate and hand the crown to his son, according to a poll released on Sunday, the monarch's birthday, that also showed his popularity slumping to a record low.

King Juan Carlos, who has been on the throne for 38 years, was once one of the world's best-loved sovereigns, respected for his common touch and for helping guide Spain to democracy in the 1970s after the death of dictator Francisco Franco.

But Spaniards have become increasingly frustrated by a long-running corruption investigation into the king's daughter, Princess Cristina, and her husband Inaki Urdangarin — particularly at a time of economic crisis and widespread unemployment.

Urdangarin has been charged with embezzling 6 million euros in public funds, though both deny any wrong-doing.

The king's approval rating also took a big hit in April 2012 when he fell and broke his hip during an elephant-hunting safari in Botswana — a lavish privately funded trip that was secret until his accident and came at a time of particularly harsh public spending cuts.

Sixty-two percent of the 1,000 adults polled by telephone d said they thought the king should step down, compared with 44.7 percent a year ago, according to the Sigma Dos poll published in El Mundo newspaper.

Only 41.3 percent of those polled had a good or very good opinion of the king, down from more than 76 percent two years ago.

Younger Spaniards, who were not alive during the Franco years, were overwhelmingly in favor of abdication, the poll showed.

Prince Felipe, 45, held on to a positive rating of 66 percent, and most of those polled said the monarchy could recover its prestige if he took the throne.

A series of hip and back operations and other health problems have fueled speculation the king, 76, might abdicate, but in his annual Christmas Eve speech, he reiterated that he was not contemplating such a move.