The former monarch did not hold royal immunity from prosecution, judges said.
The former Spanish king Juan Carlos I could face trial for alleged harassment, a British court has ruled.
A judge ruled on Thursday that Juan Carlos did not hold royal immunity from prosecution as the claim involved private acts.
The former Spanish ruler has been accused of harassing his former lover -- Danish socialite and businesswoman Corinna zu Sayn-Wittgenstein -- for years, between 2012 and 2020.
Sayn-Wittgenstein, who lives in Monaco and London, split from the former Spanish king in 2009 following increased media and public scrutiny.
High Court Justice Matthew Nicklin ruled that because her claim against Juan Carlos involves private acts in the aftermath of a romantic relationship, he does not have legal immunity under either Spanish or British law.
"Whatever status the defendant has under Spanish law and the constitution, he is no longer a 'sovereign' or 'head of state' conferring personal immunity," he ruled.
Corinna zu Sayn-Wittgenstein -- previously known as Corinna Larsen -- had an "intimate romantic relationship" with the former monarch between 2004 and 2009.
In 2020, the couple were investigated by judicial authorities over financial dealings, including assets kept in tax havens.
Sayn-Wittgenstein then sued Juan Carlos, accusing him of allegedly orchestrating threats after their break-up and ordering unlawful surveillance starting in 2012.
The socialite has asked the court for a restraining order against the former monarch, as well as compensation for her "distress and anxiety".
Robin Rathmell of Kobre & Kim -- the law firm representing Sayn-Wittgenstein -- said Thursday’s ruling demonstrated that Juan Carlos “cannot hide behind position, power, or privilege”.
“This is the first step on the road to justice; the appalling facts of this case will finally be brought before the court,” Rathmell said in a statement.
Juan Carlos’ Spanish lawyers have said that they will appeal the London court’s ruling "as soon as possible".
The 84-year-old stepped down as Spain's king in 2014, allowing his son to become King Felipe VI. He has been living in exile in the United Arab Emirates since 2020 and has retained the title of “King Emeritus” since he abdicated.
Felipe has since tried to shield his own reign from the scandals affecting his father and other members of the royal family.
Juan Carlos recently announced his desire to return to Spain and prosecutors dropped investigations into his financial dealings.