Princess Corinna zu Sayn-Wittgenstein, a wealthy businesswoman based in Claverley, is taking the so-called 'King Emeritus' to the High Court, alleging he mounted an eight-year campaign of intimidation after their relationship broke down.
Court papers accuse Juan Carlos, 83, of using spies to place her under surveillance, to hack her phones and internet, mount a campaign of smears and issue death threats.
But it has now emerged that Juan Carlos is pleading Crown Immunity to evade the claims.
According to a national newspaper report, the former king, who abdicated in 2014 in favour of his son, is trying to dismiss the case by claiming that as a former sovereign, he cannot be sued.
The allegations have caused a sensation in Spain, where Juan Carlos, 83, faces accusations of financial corruption, which he denies. He lives in exile in Abu Dhabi.
Ms zu Sayn-Wittgenstein has a base in Claverley as well as London. Her lawyer, Robin Rathmell at Kobre & Kim, has previously said: "The fact that people from a foreign intelligence service flew to London unannounced in order to threaten a private citizen… is astonishing and demands a serious investigation."
According to the court papers, Ms zu Sayn-Wittgenstein has "been subject to a continuing threat of physical harm, trespass and surveillance".
They add: "Juan Carlos has sought to disaffect her own children, has systematically sought the breakdown of many of the Claimant's close friendships and professional associations and has sought to destroy her reputation and livelihood by spreading defamatory remarks and by vilification in the media."
Ms zu Sayn-Wittgenstein, 57, claims she suffered a bizarre night-time attack at her £6 million Chyknell Hall estate in Claverley, where extensive security was breached and nothing was taken but a hole was drilled in her bedroom window.
The Daily Mail says she also received an anonymous call in which she was told she would die in a car crash in a French tunnel.
Ms zu Sayn-Wittgenstein had conducted a five-year romance with the married Juan Carlos, from 2004 to 2009, but the court papers shown to the Daily Mail suggest the relationship ended when it became clear that he was sleeping with other women.
The couple's affair became public knowledge in 2012 when, with the relationship already over, it emerged that the two had been on safari to Botswana with Ms zu Sayn-Wittgenstein's son.