The former treasurer for the governing Popular Party (PP), Luis Bárcenas, has admitted to being the author of secret hand-written ledgers — first published by EL PAÍS in January of this year — that showed illegal donations to the party from firms and cash payments to top PP officials, including Prime Minister Mariano Rajoy.

The revelations have come from judicial sources, after Bárcenas, who is currently in preventive custody, was called to appear in the Madrid High Court on Monday by Judge Pablo Ruz, who is handling the kickbacks-for-contracts Gürtel corruption case, in which Bárcenas faces a number of charges, including fraud.

In response, Rajoy told a press conference on Monday afternoon that his government would not allow itself to be subjected to blackmail.

“The rule of law does not bow to blackmail,” he told the press after meeting with the Polish Prime Minister Donald Tusk. “On February 3, I made a public statement on the PP’s position on this matter, by which I stand,” the PP leader said, adding that he could not be expected to reply to “rumors” on a daily basis.

Bárcenas was called to appear before the judge in the light of an interview the ex-treasurer gave to the daily El Mundo, in which he admitted to the authorship of the papers. He also supplied the paper with screen shots of SMS messages in which Mariano Rajoy appears to offer his support and encouragement to Bárcenas, despite the scandal already having broken and the millions of euros that Bárcenas had stashed away in Swiss bank accounts having become public knowledge. The opposition Socialists subsequently called for Rajoy to resign, and broke off relations with the Popular Party

“The SMS ratify that the rule of law does not submit to blackmail,” Rajoy said in reference to the messages.

The conservative leader also lauded the independence of Spain’s legal institutions, which are acting “with the full collaboration” of the executive.

“The institutions have acted, are acting and will act with independence and while I remain prime minister this will continue to be the case,” Rajoy told the news conference.

Earlier Monday, Deputy Prime Minister Soraya Sáenz de Santamaría said that the fact that the ex-treasurer is behind bars as a flight risk shows that “there was no negotiation with Bárcenas.” Sáenz de Santamaría declined to answer questions on the SMS messages between Bárcenas and Rajoy.

The main opposition Socialist Party has called on Rajoy to stand down over the scandal. However, the PP leader pledged to see out his mandate, which ends in 2015. “This is a stable government,” he said. “The opposition is free to evaluate things as it deems fit. The only thing I have to say is that Spain is emerging from the crisis. I will ensure political stability, and if there are others that want to play a different game, that is their responsibility.”

Rajoy said he would not “allow” the Bárcenas scandal to divert the government away from its reform program. “This is a serious democracy with institutions that do not give in to blackmail,” he said.

In his appearance before the judge, Bárcenas apparently handed over further documents, both handwritten and in digital formats, which could prove damaging to the interests of the ruling party and its top officials.

Bárcenas is reported to have accused his predecessor as PP treasurer, Álvaro Lapuerta, of being the mastermind behind the system of irregular payments and donations. He also went on the attack against Rajoy and the PP secretary general, María Dolores de Cospedal, both of whom have consistently denied the existence of a double accounting system at the party as well as having received undeclared cash payouts.

But judicial sources report that Bárcenas, who worked for the PP for over 20 years, told Judge Ruz on Monday that he “paid Rajoy and Cospedal in cash in 2008, 2009 and 2010.”

Bárcenas said he handed over to both Rajoy and De Cospedal 25,000 euros each in 2010 as a form of bonus. They were paid, he said, in 500-euro banknotes.

In a news conference late Monday, De Cospedal “emphatically” denied receiving undeclared money in envelopes from Bárcenas, and described the former PP money man’s claims as “slander.” “All of our revenues are declared to the tax office and duly justified,” she said. Asked if she received money in envelopes from Bárcenas, De Cospedal replied: “Never.” She said she reserved the right to take legal action against Bárcenas. “I want to reiterate that I have never given in to blackmail because I have nothing to hide.”

The ledgers cover the period 1990 to 2008, with the exception of the years 1993-1996. They show cash payments made by the party to Rajoy and other top officials, including Rodrigo Rato, Jaime Mayor Oreja, Francisco Álvarez-Cascos, Javier Arenas, Ángel Acebes and De Cospedal.

The papers detail more than 30 donations to the party that were illegal, either because they exceeded the limit of 60,000 euros from a single source, or because they were made by companies or businessmen in receipt of public contracts.

Bárcenas said that in September 2011 businessman Juan Miguel Villar Mar gave Lapuerta 300,000 euros for the PP. Eight businessmen have also been indicted in the case.

While the Socialists have called for Rajoy to stand down, the opposition as a whole is divided on how to proceed. Since the PP has an absolute majority in Congress a censure motion has no prospect of prospering.