He has been idolised for 30 years, sailing on expensive yachts, racing motorbikes and enjoying fine living while receiving the sort of reverential treatment that Queen Elizabeth II could only imagine.

Now, however, there are signs that King Juan Carlos I of Spain is in danger of falling out with his subjects. Under mounting pressure from critics, the King has appointed an auditor to scrutinise the spending of the Royal Family – which is kept hidden from the public by law.

The Royal Family tried to play down the significance of the move, dismissing it as a “bureaucratic decision of an internal character”. The King’s republican critics hailed it as a breakthrough in their campaign to shine a light on the Royal Family’s spending.

“The finances of the Royal Household are today a huge black hole,” said Joan Tardà, the parliamentary spokesman for the Catalan party, Esquerra Republicana. “[But] the taboo about the monarchy is starting to disappear.”

Along with other left-leaning parties Esquerra Republicana has been campaigning to force the Royal Household to reveal how it spends its €8 million (£5.5 million) annual budget from taxpayers. Now they say that the King must go farther, making the auditor report to Parliament and paying taxes on his private income.

It was the latest indignity to be suffered by the Spanish monarch, who was crowned on the orders of the dictator General Franco upon his death in 1975. Things have not been going the King’s way lately.

Last October authorities in the Russian region of Vologda began an investigation into reports that he had shot a tame bear that had been plied with vodka to make him an easy target. The King, an avid hunter, has been accused by environmentalists of shooting protected species in the past. But the story about the killing of a drunken bear, named Mitrofan, apparently incensed the Royal Family. A spokesman dismissed the report as absurd, while refusing to discuss any details.

This year royal sensibilities were further offended when El Jueves, a weekly satirical magazine, published a crude cartoon of the Crown Prince on its cover. It depicted the heir to the throne, Prince Felipe of Asturias, having sex with his wife, Princess Letizia, and saying: “Do you realise that if you get pregnant it will be the closest thing to work I’ve done in my life?” The drawing referred to a decision by the Government to award mothers €2,500 for each child they bear. A judge ordered all copies of the publication to be seized from newspaper kiosks and told the cartoonist to appear in court.

Insulting a member of the Royal Family or “damaging the prestige of the Crown” is a crime in Spain, punishable by up to two years in jail.

The move backfired, thrusting the relatively obscure publication into the international spotlight and sparking a nationwide debate. Copies of the magazine were offered for sale on the internet for up to €2,000. The trial is due to take place later this year, and the Royal Family is bracing itself for another round of negative publicity.

A Basque senator then weighed into the debate, breaking a taboo against criticising the Royal Family by describing them as “a bunch of layabouts”.

“In Britain the Royal Family puts up with [criticism] because they live in a true democracy,” Iñaki Anasagasti wrote. “Here they are untouchable.”

When he was crowned in 1975, King Juan Carlos was dismissed by many as a political lightweight who had been raised under the dictator’s arm to perpetuate his regime. When Franco died, however, Juan Carlos had other ideas. He quietly steered the country towards democracy while keeping the far Right in check.

When a section of the Army staged a coup in 1981, the King, wearing full military uniform on live television, ordered them back to barracks. The coup fizzled and the King won the admiration of a generation of Spanish newspaper editors, who have granted him unconditionally positive coverage since.

Polls show that most Spaniards like and respect the King, who, despite his lavish lifestyle and playboy reputation, has managed to cultivate an image of a grandfather and an ordinary family man. However, in a country that has always harboured a strong republican streak, analysts say that that affection does not extend to the institution itself. Many Spaniards pointedly call themselves juancarlistas – supporters of Juan Carlos – rather than monarchists.

“There’s always been a strong republicanism in Spanish society, but until now it hasn’t been a political issue,” Alejandro Quiroga, professor of Spanish history at the University of Newcastle upon Tyne, said. “Now it is becoming one, more and more.”

The biggest worry for the King, who will turn 70 in January, is that his successor does not enjoy his level of public support. Many Madrid residents groused about the expense and inconvenience that surrounded Prince Felipe’s lavish wedding in 2004 to Letizia Ortiz, a divorced former television journalist. Others have questioned his fitness to rule.

“The Royal Family has been working extremely hard to sell Prince Felipe to the Spanish public, but with this issue of El Jueves the whole question is out again,” Mr Quiroga said. “It was the last thing the Royal Household wanted.”

The Queen costs more but we know where it goes

Cost of Queen’s official household and staff £12.2m
Duke of Edinburgh’s official expenses £400,000
Upkeep of royal properties £14.5m
Royal functions and engagements £500,000
Royal travel costs £5.6m
Expenditure met by government depts and Crown Estates £4.1m

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