Spanish newspaper El Pais published extracts from what it described as secret accounts for Rajoy's People's Party from 1990 to 2008. The accounts showed businesspeople regularly paid supplements to senior party officials' salaries for more than a decade as the country slid into a painful recession causing high unemployment rates and forcing austerity cuts, The New York Times reported.
Party secretary general Maria Dolores de Cospedal said she spoke for Rajoy when she denied the allegations.
"The People's Party has only one set of accounts and it is clean, transparent and submitted to the official accounting authority," she said. "We have absolutely nothing to hide."
Former Senate president Pio Garcia Escudero, though, backed up the accounting documents published by El Pais, saying he received one of the payments, which matched a loan he was given and later repaid to repair damage to his home after a terrorist bomb attack, The Guardian reported.
Some are calling for Rajoy's resignation over the corruption scandal.
"If this is true, then he must resign," said Pere Navarro, the socialist leader in Catalonia.