Spanish newspaper El País has published what it says are handwritten accounts showing past donations from construction firms to leading political figures in the governing party.
It is the latest chapter in a corruption scandal that has embroiled the conservative People’s Party (Partido Popular).
Its former treasurer Luis Bárcenas is already under investigation following revelations that he had amassed 22 million euros in a Swiss bank account.
Although he denies wrongdoing and stood down four years ago, further accusations will put more pressure on Mariano Rajoy. The prime minister has already ordered an internal investigation and an external audit.
He and former PM José María Aznar have reportedly threatened legal action against those making accusations against the PP.
Party leaders have denied knowledge of illegal schemes but Rajoy has said he will act if wrongful activities are proven.
The scandal has prompted anger in Spain where millions are dealing with daily hardship due to the economic crisis and austerity measures.
Earlier this month a large demonstration took place outside the People’s Party Madrid headquarters.
The Bárcenas case follows others in Spain involving bankers, politicians and local councillors.