Spain's Prime Minister Mariano Rajoy is facing renewed calls to resign after a newspaper published text messages allegedly linking him to the man at the centre of a secret payments scandal.
The El Mundo report said Mr Rajoy had sent words of support to Luis Barcenas, former treasurer of the governing Popular Party (PP).
Mr Barcenas is in custody facing trial for corruption and tax fraud. He denies the allegations.
Mr Rajoy, too, denies any wrongdoing.
The PP's former treasurer was due to appear before a judge at the Audiencia Nacional - Spain's national criminal court - on Monday, a week after he admitted for the first time that handwriting in a ledger detailing payments belonged to him.
A series of newspaper allegations that Mr Rajoy and other top politicians received illicit payments has enraged a country in the depths of recession and record unemployment.
It is claimed that Mr Barcenas ran a PP slush fund that took donations from construction magnates and distributed them to party leaders in cash.
El Mundo newspaper said last week it had delivered documents with Mr Barcenas's original ledger entries to the national criminal court.
Another Spanish paper, El Pais, published similar documents earlier this year.
The prime minister is expected to face questions about the scandal from reporters when he gives a news conference with visiting Polish PM Donald Tusk on Monday.
The leader of the country's main opposition Socialist Party, Alfredo Perez Rubalcaba, called for Mr Rajoy's immediate resignation on Sunday "given the unsustainable political situation in Spain".
Luis Barcenas arrives for questioning in Madrid, 6 February Luis Barcenas has been denied bail pending his trial
"Mr Rajoy's conduct in this situation can be summarised quite simply: silence, lies, and after what we have learned today, collusion, extremely serious collusion," Mr Rubalcaba said.
El Mundo's most recent report includes a text message Mr Rajoy apparently sent to Mr Barcenas in January this year - when the slush fund allegations broke.
He said: "Luis, I understand. Stay strong. I'll call you tomorrow. A hug."
The paper said the conversations showed Mr Rajoy maintained "direct and permanent contact" from at least May 2011 to March 2013.
An editorial in the Spanish daily newspaper El Pais on Monday demanded an explanation from Mr Rajoy.
"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."
However, correspondents say it is unlikely that Mr Rajoy will step down given his party's outright parliamentary majority.
Mr Barcenas is being investigated over allegations he stashed up to 48m euros (£41m) in secret Swiss bank accounts. Prosecutors allege that some of the funds stem from illegal party donations or kickbacks.
He and his wife are also suspected of falsifying documents on their tax statements between 2002 and 2006.
The couple deny the charges.
In June, a judge ordered Mr Barcenas to be held in jail until his trial starts after prosecutors argued that he was a flight risk.