Scandal behind Queen Letizia's rarely-pictured 16-diamond engagement ring

The couple had been spotted shaking hands at the Prince of Asturias Awards in November 2003. Little did bystanders know that they were romantically involved, and would reveal their marriage plans just days later.

Two days after announcing their engagement, Felipe and Letizia spoke to the press about their low-key relationship in the Residencia del Príncipe.

"I understand the surprise that this decision caused nearly everyone, but it is a mature decision, the fruit of very intense reflections and covered with the weight and the sureness of the deep love that we have ourselves and of the course that we’ve undertaken together. 

"From when I was very young I was always a journalist and until the 31 years that I am now, I have exerted my profession with desire, dreams, determination and in that same way I now confront what we started now, with responsibility, vision and hard work to serve Spain and the Spaniards," Letizia said at the time.

During their engagement solemnisation on 6 November at El Pardo Palace, they reiterated that they made the decision to marry "some time ago" but wanted "more time" to process the news privately. It was here that Letizia showed off her white gold eternity ring, which was one of Felipe's gifts to her.

The royal described it as "a modern design" before adding: "I got cuff links made of sapphires and white gold."

Instead of a large central diamond like Meghan Markle's rock from Prince Harry or a colourful stone like Kate Middleton's iconic sapphire from Prince William, Letizia was given 16 sparkling baguette-cut diamonds that snaked around her finger.

Crafted by Spanish brand Suárez Jewellery, The Express reported it was worth £20,000. She is often spotted without the dazzling accessory on her ring finger during public appearances. This is not unusual, since many royal ladies have ditched their wedding and engagement rings in favour of dress jewellery in the past.

However, authors Esteban Urreiztieta and Eduardo Inda suggested in a biography called Urdangarin that it could be because of its connotations with King Felipe's brother-in-law, Iñaki Urdangarin.

The story goes that Felipe asked his sister Princess Christina's husband to collect the ring from the jewellery shop in Barcelona in order to keep his proposal under wraps.

Iñaki – who is now in prison – reportedly paid for it using a credit card belonging to the Noos Institute, which later came under scrutiny for embezzling public funds. The Spanish royals have not commented on the rumours.

King Felipe and Queen Letizia got married in Madrid's Almudena Cathedral on 22 May 2004. The big day saw Letizia become a princess – and then, a decade later, when Felipe ascended the throne, a queen.

The bride's elegant wedding dress was designed by 87-year-old Spanish couturier Manuel Pertegaz. Perhaps the most striking feature of the off-white silk dress was a high-standing collar. The collar, like the edging of the rest of the dress, was picked out with heavy white embroidery depicting the fleur-de-lys and ears of wheat – details from the crest of the Prince of Asturias.

Behind the bride swept a four-and-a-half-meter circular train and she also wore a diamond tiara which Queen Sofia, her mother-in-law, had worn on her wedding day to King Juan Carlos in 1962.

Special guests who received a royal invite included Queen Maxima of the Netherlands, Crown Princess Mette-Marit, then-Prince Charles and Prince Albert of Monaco.