SEVILLE, SPAIN — Prime Minister Mariano Rajoy is expected to break his silence on Saturday over a report this week alleging that he and other conservative politicians had received regular payments from a previously undisclosed account run by treasurers of his Popular Party.

According to the newspaper El País, the payments were made to Mr. Rajoy and other leading party members from 1990 to 2008 — when Spain’s construction bubble burst — via a slush fund administered by former party treasurers, including Luis Bárcenas, whom Swiss authorities recently reported to have maintained as much as €22 million, or $29 million, in Swiss bank accounts.

On Friday, El País reported that more than €5 million of the €7.5 million listed as payments to party leaders in accounting ledgers prepared by Mr. Bárcenas, copies of which were published by the newspaper, may have exceeded the legal limits under the law that was in effect at the time.

Spain’s attorney general, Eduardo Torres-Dulce, said late Thursday that the judiciary was considering incorporating the bookkeeping evidence into an ongoing investigation into possible kickbacks received by conservative politicians.