MADRID—Spain's National Court has opened a probe into allegations of slush fund payments by the ruling Popular Party's former treasurer to leading members, including Prime Minister Mariano Rajoy,

The court said Thursday that Judge Pablo Ruz has asked prosecutors for documents published recently by leading newspaper El Pais that allegedly showed lists made by former treasurer Luis Barcenas of payments to members and donations from companies over about 20 years.

Ruz last week withdrew Barcenas' passport as part of a related probe into how he once amassed (EURO)38 million ($49.53 million) in Swiss bank accounts.

The Barcenas scandals have rocked Rajoy's party although the party denies all wrongdoing.

A government poll released Wednesday shows Spaniards now rank corruption as the country's second biggest problem after unemployment.