Prime Minister Mariano Rajoy has come under growing pressure since the newspaper El Mundo published alleged secret accounting documents that indicated he had received illegal cash payments from the treasurer of his People's Party (PP), Luis Barcenas, while serving in a previous government.

"It is untenable for the prime minister to remain in office a moment longer," Socialist leader Alfredo Perez Rubalcaba said on Sunday.

The PP denies reports alleging the existence of a slush fund, in which bribes from construction companies were reportedly channeled to Rajoy and other party leaders for nearly two decades.

Barcenas is due to testify in court on Monday. He is charged with tax fraud, bribery and other crimes and was jailed last month as a flight risk. He gave an interview to El Mundo last week and handed over what he said was the original ledger detailing payments to party officials, including Rajoy.

On Sunday, El Mundo published an alleged transcript of text messages between Rajoy and Barcenas, in which the prime minister offered his support after prosecutors filed charges against Barcenas. Rajoy's text messages place him uncomfortably close to Barcenas: "Luis, nothing is easy but we'll do what we can. Cheer up," said one message from 2012.

In January, when the slush fund allegations broke, Rajoy sent Barcenas a message saying, "Luis, I understand. Stay strong. I'll call you tomorrow. A hug."

"There have been lies, resounding silence in terms of explanations and collusion, as shown by these text messages," Socialist leader Rubalcaba told a news conference.

A spokeswoman for Rajoy declined to comment on the report.

jm/jr (dpa, Reuters, AP)