Corinna zu Sayn-Wittgenstein-Sayn, 58, is seeking personal injury damages from the 84-year-old former monarch, who ruled Spain from 1975 until his abdication in 2014.
The British resident has accused Juan Carlos, who now lives in the United Arab Emirates, of spying on and harassing her after their relationship soured in 2012.
She filed a harassment suit in London in 2020, alleging he pressured her to return gifts worth 65 million euros ($65 million), including works of art and jewellery.
Juan Carlos, listed in court under his full name Juan Carlos Alfonso Victor Maria De Borbón y Borbón, has not appeared at hearings so far and strenuously denies any wrongdoing.
In March, the High Court in London rejected Juan Carlos’s claim that English courts had no jurisdiction to hear the case because he has state immunity as a royal.
Judge Matthew Nicklin said that “whatever special status the defendant retained under the law and constitution of Spain, he was no longer a ‘sovereign’ or ‘head of state’ so as to entitle him to personal immunity”.
The former king’s lawyers appealed and won permission for a legal challenge concerning the period when Juan Carlos was on the throne.
This will be examined by three judges at the Court of Appeal from 1030 GMT on Tuesday, with a ruling expected in a few weeks, after which the harassment lawsuit will continue.
The hearings resume as Zu Sayn-Wittgenstein-Sayn has been discussing the relationship in a recent podcast called “Corinna and the King”.
Court submissions claim Juan Carlos, who is married, was in an “intimate romantic relationship” with the divorcee from 2004 to 2009 and showered her with gifts.
She alleged that Juan Carlos began harassing her after their relationship broke down, using threats, break-ins at her properties and surveillance.
The podcast is available on Spotify here and other platforms.
Juan Carlos “demanded the return of gifts”, she claimed, and she suffered “trespass and criminal damage” at her home in rural central England.
Gunshots were fired at and damaged security cameras at the front gate of the property, she alleged, accusing the former king of being angry at her refusals.
The couple’s relationship became known in 2012, when the monarch broke a hip while on holiday in Botswana with Zu Sayn-Wittgenstein-Sayn and had to be flown home, sparking public anger during a period of record unemployment in Spain.
Two years later, dogged by the scandals and health problems, Juan Carlos abdicated at the age of 76 in favour of his son Felipe VI, who has now distanced himself from his father.
Juan Carlos went into self-imposed exile in the United Arab Emirates in 2020.
The pair attended the state funeral of Queen Elizabeth II in September and were seated together.
Juan Carlos was protected for decades by his huge popularity as a key figure in the democratic transition following the death of dictator Francisco Franco in 1975.
The excesses of the monarch only came to light in the last years of his reign, triggering a string of investigations over corruption scandals.