As he basks in the warm glow of respect, and even affection, surrounding his coronation this weekend, King Charles should recall the story of his distant cousin and near namesake ex-king Juan Carlos of Spain as a warning of how speedily a popular monarch can go from hero to zero.

The ties between Spain’s Royal Family and our own are close in blood and warm mutual regard. Both families are direct descendants of Queen Victoria, with Spain’s current King Felipe calling our late Queen Elizabeth ‘Aunt Lilibet’; our new monarch reportedly had a private lunch with Felipe’s disgraced dad Juan Carlos only last month.

The sad downfall of Juan Carlos should flash a red warning light to our somewhat naive new King

When he succeeded Spain’s durable fascist dictator General Franco as head of state in 1975, few thought that Juan Carlos would last for long. His Bourbon family had a track record of alienating their subjects, and the king’s grandfather King Alfonso XIII had been forced from his throne to make way for a republic in 1931.

But the new king confounded these dire expectations. Brought up by Franco as almost an adopted son with the aim of perpetuating his dictatorship, instead Juan Carlos lost no time in dismantling the old regime and introducing Spain to the unfamiliar concept of democracy. He legalised leftist parties that Franco had banned, lifted censorship, and in 1981 was instrumental in putting down a military coup by diehard Franco loyalists who had seized Spain’s parliament, the Cortes, and held politicians hostage.

The king won universal golden opinions for a wisdom belying his inexperience, and the monarchy hit new heights of public popularity. Then it all went horribly wrong. Things began to unravel in April 2012. The king left Spain on an ecologically incorrect trip to shoot elephants on a safari in Botswana at a time when his country was in the throes of an economic crisis, with 50 per cent youth unemployment.

The safari attracted attention because the now ageing monarch fell and broke his hip during it. Then it was revealed that the jaunt had cost more than €40,000 – a tab mysteriously picked up by a Arab financier. Moreover, the king was not alone during his jumbo slaughtering trip. His companion was a Danish-born German blonde entrepreneur exotically named Corinna zu Sayn Wittgenstein-Sayn. The king’s long-suffering wife, Queen Sofia, signalled her disapproval of the scandal by waiting three days before visiting her faithless husband in hospital.

The king’s sky high popularity nosedived as more evidence of his dodgy financial affairs and his astonishing amatory adventures leaked out. (An aptly named real life Don Juan, the monarch was rumoured to have enjoyed relationships with no fewer than 5,000 women during his reign).

As a result, in 2014, he abdicated in favour of his son Felipe VI. Instead of making things easier for his heir, however, Juan Carlos continued to behave with Bourbon arrogance, embarrassing his family by accepting large amounts of cash from foreign sources that he had failed to disclose in order to avoid paying taxes.

When prosecutors began investigating his finances in 2020, the ex-king seemingly panicked and fled into exile, taking up residence with his friends in Abu Dhabi in the United Arab Emirates. Since then he has only twice returned briefly to his homeland, once for his sister’s funeral and once to take part in a yachting regatta last year.

Meanwhile another set of scandals arose. Corinna, who had by this time been replaced in favour of a new mistress, began legal action against Juan Carlos. She alleged harassment against her former royal lover, now 85 and walking with a cane after hip replacement surgery.

In court documents she told a truly astonishing tale of the ex-king’s alleged antics. During their five year affair, she claimed, Juan Carlos had lavished gifts of cash, jewellery and other valuables on her worth €65 million (£57 million) – which, it has been claimed, he later demanded be returned. She subsequently alleged that Spanish state secret service agents acting for him threatened her with dire consequences and even broke into her Shropshire country home to intimidate her.

Juan Carlos has denied the allegations. He has maintained that, in any case, any such actions had occurred while he was still king and that, as a sovereign, he was therefore immune from prosecution. A final judgement on the case is expected later this year.

But, it may be asked, what has this rather Gothic tale of regal misdeeds got to do with the stable home life of our own dear king? Surely Charles has demonstrated over many decades his monogamous fidelity to Queen Camilla.

While this may be so, the King does share an unwise propensity with his distant Hispanic cousin to hobnob with wealthy potentates from the Middle East only too eager to lavish cash on European royalty. Last June, the Sunday Times reported that Charles had personally accepted €3 million in 2015 in a series of meetings with the former premier of Qatar, the immensely rich Sheikh Hamad bin Jassim al-Thani. (A spokesperson for Charles said the money was immediately passed to one of his charities and that all processes had been followed)

Piquantly, the paper reported that the cash was contained in a suitcase and a bag from Fortnum and Mason, the upmarket purveyors of foodstuffs on Piccadilly to the Royal Household. It was allegedly counted by Royal aides before being processed by Coutts, the Royal family’s bankers and passed to one of Charles’s favoured charities.

The sad downfall of Juan Carlos – let alone the relationship between his own brother Prince Andrew and the wealthy paedophile Jeffrey Epstein – should flash a red warning light to our somewhat naive new King: take care of those you mix with, and, before taking money from anyone, ask yourself what is in it for them. When it comes to cash in plastic bags, the irreproachable example of his mother rather than Juan Carlos’s blighted Spanish reign should be Charles’s guiding light.