Lawmakers in the lower house, including those in the PP, voted Thursday to create an investigative commission into the allegations, as proposed by the Socialists and two other opposition parties.
The PP has been hit by a series of corruption scandals over the years, and several former lawmakers and civil servants from the party are currently on trial over claims that companies showered them with bribes to win public contracts.
Most of the defendants are facing criminal charges, but the party itself is accused only of having benefitted from funds obtained illegally, making it liable to civil penalties.
But during the hearings, Luis Barcenas, a former party treasurer on trial, said publicly for the first time that the PP once had a slush fund, a claim that is now being investigated by judicial authorities.
In their proposal to create the commission, the three opposition parties said it would investigate the possible "existence of different networks of irregular donations, the awarding of public contracts and other mechanisms of irregular funding."
Corruption is a major issue in Spain, where the Socialists and regional politicians have also been hit by scandals.
Such is public anger over the issue that many voters have flocked to two relatively new parties -- the far-left Podemos and the centre-right Ciudadanos.
As a result, although the PP still won general elections last year, it failed to get the absolute majority it won in 2011, and Rajoy is now the head of a minority government.