Spanish opposition leaders on Sunday urged Prime Minister Mariano Rajoy to resign after a newspaper published what it said were text messages Rajoy sent to his party's former treasurer after the treasurer was accused of making slush fund payoffs to politicians including Rajoy.

The ongoing scandal of alleged secret payments has shaken Rajoy's governing Popular Party and damaged its popularity ratings. The publication of the text messages by the El Mundo newspaper placed even more pressure on Rajoy, whose resignation was demanded by leaders of two key opposition groups.

The El Mundo report showcased what it identified as text messages over a period of three years between Rajoy and former Popular Party treasurer Luis Barcenas, who was a key player in the party's financial division for two decades.

Barcenas, jailed last month while he awaits possible trial on tax fraud and money-laundering charges, told El Mundo last week that the party has long been illegally financed. El Mundo said documents show Rajoy received 42,000 euros ($54,000) in payments while serving as a minister between 1997 and 1999.

The documents El Mundo published look like account ledger sheets that it says are genuine and show what it alleges is Rajoy's name next to payment amounts. What appear to be the same ledgers were previously — some months back — also published by another leading newspaper, El Pais.

The striking resemblance between the ledgers has led many observers, and opposition leaders, to give credence to the reports.

Rajoy and members of his party have denied wrongdoing, but the allegations have been huge news in Spain, where people are coping with harsh austerity measures and tough economic reforms aimed at reducing debt amid unemployment of 27 percent.