"Spain and the Spanish people need our party (the PP) to be clean," the outspoken Aguirre said during a press conference at PP headquarters in Madrid.
"If there have been corrupt (officials), we have to find out who they were and denounce them more roundly than the courts are able to do," said the influential party figure in words directed towards some 300 members of the Madrid PP's Executive Committee.
"If there have been funding irregularities", we have to "recognize them", "explain them", and "present them to the people sincerely and honestly", said Aguirre, breaking ranks with her party which has issued only a brief statement denying any knowledge of the slush fund.
Aguirre made no reference to the PP's ex-treasurer Bárcenas during her press conference, but she was clearly referring to the former top official's claims he operated a second set of books for the party.
On Tuesday, Spain's El Mundo newspaper published what it purported to be an original handwritten accounts ledger by Bárcenas showing illegal payments to Spanish Prime Minister Mariano Rajoy while he was a minister.
El Mundo said the ledgers were compiled in part by the party's former treasurer Luis Barcenas, who is now behind bars facing corruption charges.
The PP has hotly repeated its denial of any illegal payments to its members and has distanced itself from Barcenas.
On Tuesday night, several hundred protestors gathered outside the PP's Madrid headquarters calling for the resignation of Rajoy.
They later moved onto the city's iconic Puerta del Sol square.