Spain's state attorney says there is sufficient cause to investigate fresh allegations of irregular financing of Spain's governing Popular Party and that if necessary Prime Minister Mariano Rajoy would be called in for questioning.

Speaking on 13TV television late Thursday, Eduardo Torres-Dulce was referring to El Pais' publication of the so-called "secret papers" of former party treasurer Luis Barcenas, allegedly documenting systematic payments of under-the-table money to leading members, including Rajoy over many years. The money was allegedly paid by businesses, many in the construction sector.

Torres-Dulce said a decision on opening a fresh investigation or possibly incorporating the allegations into one already existing will be taken within days.

El Pais on Friday said nearly two thirds of the payments were in violation of Spain's old party financing law.