The trial centres on an alleged system of parallel accounting used to pay bonuses to People's Party leaders and collaborators.
Former Spanish prime ministers Mariano Rajoy and José Maria Aznar will testify as witnesses on Wednesday at a trial over an alleged party slush fund.
Both Rajoy and Aznar will answer questions from the National Court of Justice via video conference.
The trial focuses on an alleged system of parallel bookkeeping by the right-wing People's Party (PP) to manage undeclared funds.
The accounts were run by Luis Barcenas who served as party treasurer between 1990 and 2009 while Aznar and then Rajoy led the PP.
The trial opened on February 8 and will run until May.
'Envelopes of cash'
For nearly 20 years, the alleged slush fund was fed by donations from businessmen and allegedly used to pay bonuses to PP leaders and collaborators.
It also funded the renovation of the party’s Madrid headquarters, Barcenas has said.
Rajoy has always denied any knowledge of the system. But Barcenas has testified he was “perfectly aware” of it, telling the court earlier this month that he was one of those who received envelopes of cash in the period before he became prime minister.
Aznar denied the existence of the system of illicit accounting as he appeared before a parliamentary commission of inquiry in 2018.
El Pais newspaper first published details of the accounts in 2013 in so-called "Barcenas papers".
Public prosecutors have requested a five-year prison sentence for Barcenas over the slush fund.
He is currently serving a 29-year sentence over a separate case centred on a system of bribes given to former party officials in exchange for public contracts between 1999 and 2005.
Rajoy appeared in court in 2017 as part of that trial, becoming the first sitting prime minister to take the witness stand in the country’s modern history.
The following year he became the first prime minister in Spain’s recent history to be ousted from office in a no-confidence motion triggered by the long-running corruption scandal.
Current PP leader Pablo Casado says neither he nor the party’s current leadership can be blamed for past wrongdoing.