Updated

Spanish Prime Minister Mariano Rajoy faced calls to explain himself or resign over his alleged support for the ruling Popular Party's disgraced former treasurer, who headed to court Monday over a slush fund scandal.

The 58-year-old, grey-bearded premier has denied any wrongdoing and refused to comment in past weeks on the growing controversy centred on former party treasurer Luis Barcenas.

Pressure on Rajoy mounted, however, as more allegations were revealed and as the 55-year-old Barcenas faced a High Court judge to answer questions over secret political payments.

Barcenas was called to appear in the Madrid court after conservative daily El Mundo last week published what it said was an original page from Barcenas' slush fund ledger and delivered the document to the court.

The excerpt purportedly showed extra payments from a secret fund to party officials including Rajoy when he was a minister under then prime minister Jose Maria Aznar in 1997, 1998 and 1999.

Barcenas is suspected of running a slush fund financed by corporate donors who were then rewarded with public contracts. The cash was allegedly used to supplement senior party members' salaries.

In the latest blow to Rajoy, the conservative daily El Mundo on Sunday published friendly mobile text messages between the prime minister and Barcenas from May 2011 to March 2013, ending some two months after the scandal erupted.

"Luis, I understand, be strong. I will call you tomorrow. Best wishes," said one of the messages reportedly from Rajoy to Barcenas, dated January 18 when El Mundo first published allegations over the slush fund.

"It is not good to try to determine what we will say or to comment on things that must be presented to the courts, which we must all respect," read another message allegedly sent by Rajoy.

Barcenas reportedly told El Mundo in an interview published July 7 that the Popular Party had engaged in illegal financing for nearly 20 years.

The Popular Party has repeatedly denied secret financing allegations.

The leader of Spain's main opposition Socialist Party, Alfredo Perez Rubalcaba, on Sunday accused the premier of "serious collusion" with Barcenas and said he was severing all contact with the prime minister and his party.

"Given the unsustainable political situation in Spain, the Socialist Party calls for the immediate resignation of Mariano Rajoy as head of the government," he said.

But few people in Spain expect Rajoy to step down given his party's outright parliamentary majority.

An editorial in leading daily El Pais on Monday demanded an explanation from the premier.

"Out of respect for the democratic system, the citizens and his own party and voters, the head of government must give a true explanation to parliament," it said.

"Otherwise it will be impossible for him to regain his credibility."

Rajoy has so far resisted calls to appear before parliament over the scandal and has carefully avoided even mentioning the name Barcenas.

But he is expected to face the press Monday after hosting a visit by Polish Prime Minister Donald Tusk.

The corruption allegations have outraged Spaniards suffering in a recession with a record unemployment rate of more than 27 percent. Dozens of protesters rallied outside the Popular Party headquarters in Madrid on Sunday.

Barcenas is already behind bars while under investigation in a separate graft case.

A Spanish judge remanded Barcenas to custody on June 27 over alleged money laundering and tax fraud. He said the move was aimed at preventing him from fleeing and to preserve evidence.

Barcenas is being investigated over tens of millions of euros he allegedly stashed in Swiss bank accounts.