The bill came in at €5m. It was unaffordable for someone whose official stipend was around €200,000 a year, €140,519 after tax.

Out of a sense of “family honour”, Prince Álvaro de Orleans, a distant Spanish cousin living in Monaco, paid the bill. He paid through his Zagatka Foundation, registered in the secretive tax haven of Liechtenstein, and with bank accounts at Credit Suisse and Lombard Odier in Switzerland. Juan Carlos’s name never appeared on the bookings.

But it was Juan Carlos’s address – the Zarzuela Palace in Madrid – that de Orleans sometimes put down on booking contracts. One flight was even booked in the name of Juan Carlos’s head of security and set to depart from a military airport outside Madrid.

Despite these clues that de Orleans was fronting for Juan Carlos, no one at AirPartner, the British private jet company – which has an obligation to check the source of clients’ funds, especially when they are “politically exposed persons” – seemed to notice the flights were really for a head of state with an income too low for them.

When de Orleans was asked about the origins of some of the money in Zagatka – not by AirPartner but by a Swiss prosecutor – he said a real estate deal at Playa del Carmen in Mexico. But he couldn’t remember the name of the Caribbean company he dealt with, nor the name of its chief executive, nor who signed his cheque, nor how many hours he had worked on the deal.

Another thing he didn’t remember was a 2010 transfer of €150,000 to the HSBC Monaco account of Corinna zu Sayn-Wittgenstein, who had begun an affair with Juan Carlos in 2004 that she said ended in 2009. The Swiss prosecutor also asked her about Zagatka.

“For me, the Zagatka Foundation is the foundation of Juan Carlos,” Sayn-Wittgenstein said. “It is wrong that this foundation is registered in the name of his cousin Alvaro.”

This is the problem with estimating Juan Carlos’s personal fortune: his wealth is hidden behind administrators and in secretive jurisdictions.

It has never stopped people from trying – estimates of $50m to $2bn go back decades – and people have begun trying again since the press discovered that, in 2012, Juan Carlos gave Sayn-Wittgenstein an “unsolicited gift” of €65m “out of love and gratitude”. 

When Tortoise asked Sayn-Wittgenstein whether the “gift” was a lot of money for Juan Carlos, she said, “Well, he’s, you know, allegedly worth about two billion. And these estimates are made by very serious publications.”

The serious publications that Sayn-Wittgenstein referred to are the New York Times and Forbes. Over and over, people say that these publications have estimated Juan Carlos’s wealth.

Here, for example, is a spokesman for the United Left party, which is part of Spain’s coalition government: “We read in Forbes that Juan Carlos has a personal fortune of 1.8 billion euros, and we don’t know what the origin of that fortune is.” 

Wherever they got the number, it wasn’t from Forbes. When Tortoise asked the business magazine to confirm its estimate of Juan Carlos’s wealth, it replied: “We are unable to confirm King Juan Carlos’ net worth as he has not been featured on a Forbes wealth list.”

The New York Times, it turns out, never estimated Juan Carlos’s fortune either. 

In 2012, when Juan Carlos was at the centre of his elephant-hunting scandal, the newspaper’s Spanish correspondent wrote that, “The Spanish royal family’s wealth has been estimated at up to $2.3 billion, a sum that supporters contend was inflated by the inclusion of government properties”.

The correspondent didn’t respond to a request for the source of the $2.3bn number. Someone close to the Spanish royal family tells Tortoise that they “pulled that figure out of a hat”.

Actually, they likely pulled it out of EuroBusiness, a magazine founded by Bernie Ecclestone, the former chief executive of Formula One. Before a punishing defamation lawsuit ended the magazine, it produced a European rich list.

In its 2002 rich list, it estimated Juan Carlos’s wealth at €1.79bn. At the exchange rate in 2012, the year the New York Times piece came out, that number becomes $2.3bn. The newspaper did say that “supporters” think the number was inflated by the inclusion of government property.

It was. Already in 2002, the Spanish ambassador in London, the marquis of Tamarón, wrote to EuroBusiness: “The absurd figure of 1,700 million euros can only be explained by having mistakenly understood that the public assets owned by the National Heritage of the Spanish State are the private property of His Majesty the King, which is obviously inaccurate.”

The Spanish royal family is, in fact, one of the cheapest in Europe. 

It gets around €8m from the public budget. The money pays for royal stipends, like the €200,000 Juan Carlos used to get, but also maintenance of public palaces. 

In comparison, King Willem-Alexander of the Netherlands is expected to receive a personal salary of €1m this year, while Queen Elizabeth received £85.9m from the taxpayer last year. 

Public money, then, can’t account for the Spanish royal’s potential billionaire status. 

Another income source could be the inheritance left to Juan Carlos by his father, Don Juan de Borbón, after he died in 1993. Even this is controversial.

Don Juan was long rumoured to be on the brink of poverty; he lived most of his life in exile, in Italy, Switzerland and Portugal, and never ruled. It wasn’t until 2013, when Juan Carlos was embroiled in corruption scandals, that Don Juan’s inheritance was revealed by El Mundo. 

And it was a hefty sum of money. Based on documents from a Swiss bank, the newspaper revealed that Juan Carlos had been left about 370 million pesetas – €2m in 1993 – thanks in large part to the sales of properties that, the newspaper alleged, had been expropriated from the monarchy.

Juan Carlos immediately transferred the amount to a bank account abroad, and the money remains undeclared in Spain.

Then there is the oil. Even before he became king in 1975, it’s reported that Juan Carlos travelled often to the Middle East, where he was popular, to mediate with heads of states. 

Spain’s energy model largely depends on crude oil imports. During the 1973 oil crisis, when members of the Organization of Arab Petroleum Exporting Countries proclaimed an oil embargo on countries that supported Israel during the Arab-Israeli war, Spain’s dictator Franco asked Juan Carlos to mediate.

During this time, he built relationships with Middle Eastern monarchs, relationships that he solidified during the second oil crisis of 1979. Records show that he was able to obtain large amounts of oil for Spain at well below market price throughout the 1970s.

It is reported by, among other publications, Spanish Vanity Fair that Juan Carlos received kickbacks or commissions for each of these oil deals – though there is no formal record of this. That, the rumours say, is how he began to build his wealth. The number, if true, is likely to be in the low millions. Even if this was intelligently invested abroad, it’s unlikely to have reached the billions.

Beyond his oil commissions, Juan Carlos’s relationship with Arab monarchies has generally been at the centre of the discussion around his wealth. 

Over the years, the king has received hundreds of gifts from his fellow royals. These range in scope and value: in the late 1970s, Jordan’s King Hussein built La Mareta, an island home in Lanzarote that he later gifted to the king, and where Juan Carlos’s mother Doña María de las Mercedes would pass away in 2000.

Other gifts were more traditionally opulent. He received two model FF and F40 Ferraris – worth €300,000 and €600,000, respectively – from Dubai’s Sheikh Mohamed bin Rashid Al Maktoum. He got a gun emblazoned with precious gemstones from King Hussein. There was also the 95-foot yacht, the Fortuna, gifted by King Fahd of Saudi Arabia.

There are rumoured injections of cash, too: Juan Carlos’ former wealth manager says he once picked up a $1.9 million briefcase of money from the Sultan of Bahrain in 2010. The same wealth manager facilitated a transfer of $100m from the Saudi Ministry of Finance to Juan Carlos’s Swiss bank account in 2008. It was described to him by Saudi’s ambassador in America as “a pure gift”.

In 2015, the current King Felipe, Juan Carlos’s son, stipulated that the royal family must not accept gifts that exceed “habitual use, social use, or courtesies”. And yet, as recently as 2018, Juan Carlos received cufflinks, a watch, a ring and a fountain pen from Saudi Arabia’s de facto ruler Mohammed bin Salman.

The extent of his gifts, once again, is unknown. Many are now being revealed as part of the corruption investigations into the $100m “pure gift”.

Like many before us, we failed to estimate a figure for Juan Carlos’ personal fortune. The limited evidence out there suggests, though, that $2.3bn is too high.

Juan Carlos is now 83: he would have had to earn $28 million a year, every year of his life, to get to a sum of $2.3bn. 

With that in mind, a gift of $65 million to Sayn-Wittgenstein seems extraordinarily generous. Juan Carlos’s wealth manager did say that the former king was “very generous”.

His subjects seem less sure. While Juan Carlos ordered €5.5m in cash withdrawals from his Swiss bank account between 2008 and 2012 for what his banker said was “personal use”, Spain’s unemployment rate went from 11 to 25 per cent. Some 40 per cent of the country now wants a republic.

Photographs by Dnphotography/Sipa/Shutterstock, Action Press and Getty Images