The chief judge of the Constitutional Court, Francisco Pérez de los Cobos, appears on a list of donors and members of the Popular Party between 2008 and 2011.
The document, which lists members and their national identity card numbers, was submitted as part of the ongoing investigation into former PP treasurer Luis Bárcenas over party financing and alleged irregular payments to top party officials.
De los Cobos is listed as having paid an annual fee of 37.14 euros to the PP, the minimum required for party membership. Spanish law and the Constitution prohibit judges from belonging to political parties or labor unions.
De los Cobos has offered no explanation over the revelation but sources close to the chief of Spain's highest court said Wednesday that taken literally, the articles contained in the Constitution and the laws governing judicial conflict of interest prohibit a magistrate from direct involvement with a party, but not simple membership.
De los Cobos, a former lecturer in labor law at both the Autònoma University of Barcelona and Madrid's Complutense University, was put forward as a Constitutional Court magistrate in 2010 and his candidacy was supported by the PP in the Senate. He took up his post in January 2011 and was elected as the court's chief judge on June 16 of this year.
"When a case of incompatibility arises in the naming of a magistrate, before taking possession the incompatible post or function must be ceased. If this is not done within 10 days of the naming, it will be understood that the post of Constitutional Court magistrate has not been accepted," the law states.