When Juan Carlos abdicated in favour of his son, he gave up his immunity from prosecution. That leaves him vulnerable to at least two paternity suits against him.
The first was made by a Spanish waiter, Alberto Solá Jiménez who claimed that his mother may have had an affair with Juan Carlos before he married Queen Sofia.
The second has been made by Belgian housewife, Ingrid Jeanne Sartiau.
Sartiau and Solá undertook DNA tests in 2012 that showed there was a 91% chance that they shared the same parent. However, their bid to make King Juan Carlos take a paternity test failed due to his protection at that time.
Now though, with his immunity gone, the Spanish Supreme Court will be investigating the case of Jeanne Sartiau; the case for Alberto Solá Jiménez had already been dismissed because it lacked legal basis.