Corinna zu Sayn-Wittgenstein-Sayn, a businesswoman from Denmark, has brought the claim against the former king – who abdicated in 2014 – in the High Court in London, The Guardian reports.

She alleges that, after the end of their five-year affair, the king ordered Spain’s National Intelligence Centre (CNI) to harass and intimidate her and her children.

In one incident, Ms Sayn-Wittgenstein-Sayn claims that a copy of a book about the death of Princess Diana – the subject of widespread conspiracy theories – was anonymously left inside her villa in Switzerland.

Juan Carlos denies all of the allegations and is seeking to have the case dismissed on the basis of a lack of evidence and state immunity.