The news was announced by the Spanish Supreme Court.
The 48-year-old Ingrid Jeanne Sartiau claims to be his daughter and hopes DNA tests can be carried out to prove this. Sartiau took the matter to court and the Spanish court accepted that her claims would be investigated.
Juan Carlos was believed to have 20 days to respond to the paternity claims, but as no response came from him, Ingrid Sartiau also launched an appeal.
Both appeals will now be passed on to the different parties and the prosecutor. A decision is expected around mid-March, although this could not be confirmed.
The 77-year-old Juan Carlos was facing two paternity claims, one from the Belgian housewife Ingrid Jeanne Sartiau – who is in her late forties – and another from Alberto Sola Jimenez, a Spanish waiter in his late fifties.
The Spanish supreme court agreed to examine the Belgian claim last month, but dismissed the other one.
This was despite claims that both had undergone a DNA test to see whether they were related, with their DNA matching for 91 percent.
Ms Sartiau claims her mother had an affair with Juan Carlos in the 1960s when he was still a crown prince but after he had married.
Juan Carlos lost his immunity when he abdicated from the throne last summer.
The cases were brought against him while he was still on the throne, in 2012, but were then rejected because he enjoyed immunity from prosecution.
Flandersnews.be / Expatica