Tensions between the feuding members of the country's first family of showbiz blew up into full-scale war earlier this week when his children Laura Smet and David Hallyday challenged his "rewritten" will, in which he left everything to his fourth wife Laeticia, 32 years his junior.
Hallyday's 51st studio album has now become part of the spat, after the "French Elvis" spent much of 2017 recording 12 songs for the record, which remained unfinished when he died of lung cancer in December.
Smet will go to court to "get to know the posthumous album project in order to exercise her rights as heir, to be able to confirm that the artistic integrity of all its contents were respected in this album, which the press has announced would be completed soon," her lawyer Emmanuel Ravanas said on Saturday.
Smet was "mainly pre-occupied with defending her father's work," Ravanas said, adding that Laeticia had "sharply opposed" an amicable plea to end stonewalling over the finalised album, "about which she knows little".
"Johnny Hallyday had proudly listened to the first raw recordings of some songs with his daughter Laura on October 4," Ravanas added.
Along with David, Smet has urgently appealed to a court in Nanterre, to the west of Paris, to receive information about the album project within 48 hours. They also demanded Hallyday's real estate holdings, which include several luxury villas, be frozen ahead of a hearing scheduled for March 15.
The fractured family was able to maintain a unified front for the funeral of the country's biggest rock star in December, when hundreds of thousands of his fans thronged the centre of Paris for a "national popular tribute". But that uneasy truce exploded when details of the rocker's will became known on Monday.
Smet said she was "stupefied and hurt" that she and her half-brother David -- the singer's only two biological children -- had been left nothing, with everything going to Laeticia and eventually the two Vietnamese girls she and Hallyday adopted.