Jacques Boël still is Delphine's legal father. Delphine still is his heiress (knowing that Jacques Boël will do everything to shield his immense fortune from his daughter, but that aside).
The Court ruled that indeed Jacques Boël has the "Bezit van Staat"/"Possession d'État" (possession of state, a bond of paternity) over Delphine: for 51 years Jacques Boël is Delphine's legal father, a situation which is still current. This "Bezit van Staat" /"Possession d'État" is a very important basic legal situation in Belgian law.
The lawyers of King Albert argue that Delphine has to start a procedure to deny that Jacques Boël is her legal father. The Court went along with that argument but was of the opinion that the right of a child to know who is the natural father was equally strong. So the ruling came in two étappes:
Étappe 1: Albert van België / de Belgique / von Belgien has to cooperate with a DNA-test. With this Delphine's argument that she has the right to know her father is secured.
Étappe 2: the result of the DNA-test will not be disclosed until the Appeal of the King has been read in court. With this the right of Albert on a proper jurisprudence of law is secured.
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Challenging a "Bezit van Staat"/"Possession d'État" is difficult because Delphine has to argue that Jacques Boël never has been her father, that he never has executed his paternity rights and duties, etc. For Anglosaxon readers it is often hard to understand that the State is not interested in who contributed into Delphine's conception. For the State it is only relevant that in 1968 Jacques Boël came to Uccle town hall to declare that to him, in his marriage, was born a daughter, named Delphine. From then on he had "Bezit van Staat" : he executed the parental authority, he had parental responsibility over his daughter and between the two a legal familial bond has been established which is still intact to present day.