The announcement comes in the immediate aftermath of the Royal Spanish Football Federation's (RFEF) acceptance of the date proposed by bitter rivals Barca and Madrid, which the Competition Committee approved on Wednesday.
Civil unrest in Catalonia forced the first Clasico of the 2019-20 season, which was originally set to take place on October 26, to be postponed.
The clubs at the centre of Spanish football's showpiece fixture reached a quick consensus on a new date but LaLiga, responsible for administering the top two divisions, opposes December 18 as it falls in midweek.
President Javier Tebas is said to favour December 7, a Saturday, in line with the original plan for the game and LaLiga has now reaffirmed its resistance to the selected date.
A statement read: "LaLiga does not share the resolution of the Competition Committee to set December 18 - following the agreement reached by Barcelona and Real Madrid, despite LaLiga's refusal - as the date of the previously postponed Clasico.
"It is studying the documentation recently received from the file referring to this procedure to assess the possible legal actions to be taken against the corresponding institutions and before the public bodies or jurisdictional bodies that it deems appropriate."
LaLiga further questioned the "briefness of the resolution" and referred to a "lack of reference to legal norms" as potential grounds for legal action.
Earlier, RFEF said LaLiga had made numerous submissions to the rescheduling process that were deemed to be "not binding".
"At its meeting today, the Committee has also analysed the latest pleas of the clubs, who stood by their initial proposal to play the game on December 18," RFEF added.
The match was postponed last week as violent scenes broke out in Barcelona after nine Catalan political leaders received long prison sentences for their roles in the region's 2017 referendum and subsequent declaration of independence.