Confronted by an intensifying corruption scandal, Prime Minister Mariano Rajoy of Spain insisted on Monday that he would not yield to “blackmail” and resign, even as his former party treasurer reportedly acknowledged before a judge that he had made payments for years to Mr. Rajoy and other center-right politicians from a secret party slush fund.

Mr. Rajoy was responding to reports in leading Spanish newspapers that Luis Bárcenas, the former treasurer of the governing Popular Party, had made new and damaging allegations in a four-hour closed hearing on Monday. The newspapers cited judicial sources whom they did not identify.

Mr. Bárcenas has been at the heart of the scandal since January, when he was found to have amassed millions in secret Swiss bank accounts. On Monday, the newspapers said, he told the judge in charge of the case, Pablo Ruz, that he had made undeclared cash payments to Mr. Rajoy and others as recently as 2010, the year before Mr. Rajoy was elected prime minister.

The newspaper reports said Mr. Bárcenas, who previously denied any wrongdoing, had told the judge that he made payments in 2010 to María Dolores de Cospedal, the secretary general of the Popular Party. Ms. de Cospedal called the accusations defamatory.