The allegations of undisclosed payments within the Popular Party (PP) have rocked Rajoy's government as it tries to steer Spain out of a deep recession.
The National Court, Spain's highest criminal tribunal, has been investigating the 55-year-old Luis Barcenas since the leading El Pais newspaper in January published extracts of a slush fund's account ledgers it said were handwritten by him.
The ledgers supposedly showed tens of thousands of euros being received from donors, mostly from the construction industry, and then being channelled to senior Popular Party officials.
They purportedly showed payments to Rajoy between 1997 and 2008.
Examining judge Pablo Ruz summoned Barcenas for questioning on Friday but during his court appearance Barcenas said he "would never" answer the magistrate's questions, according to a judicial source who attended the closed-door hearing.
Barcenas also refused to undergo a handwriting analysis, aimed at determining whether he was the author of the ledgers, the source added.
Under the Spanish judicial system, he has the right to refuse to testify or to undergo handwriting tests.
Barcenas was the party's treasurer until 2009 and its accounts manager from 1990 to 2008, the year that Spain's decade-long housing boom went bust.
Rajoy has rejected calls to resign and denied receiving undisclosed payments, saying the supposed ledgers are false.
Barcenas denies being the author of the ledgers, which he, too, has described as a fake.
The former party heavyweight faces multiple corruption probes. He has been under investigation since 2009 in the so-called Gurtel case, a business and political corruption scandal related to contract awards, allegedly reaching into the senior ranks of the Popular Party.
In a separate corruption affair also involving Popular Party officials, the National Court last month slapped a travel ban on Barcenas after questioning him over millions of euros in undeclared funds he held in Swiss bank accounts.