Germany's President Christian Wulff is under intense pressure to give a full explanation of his conduct over a controversial home loan he received.
The popular daily Bild said Mr Wulff, an ally of Chancellor Angela Merkel, threatened it with legal action if it published details of the loan.
Mr Wulff came under a barrage of criticism in German newspaper editorials on Tuesday over the affair.
He has denied misleading state deputies over the home loan.
Mr Wulff, a centre-right Christian Democrat, was appointed in June 2010 to the largely ceremonial post of German president.
It has emerged that he received a home loan of 500,000 euros (£417,000; $649,000) from the wife of a wealthy businessman in October 2008, while prime minister of Lower Saxony state.
Bild says that last month, just before it published details of the loan, Mr Wulff rang its chief editor Kai Diekmann and left a voicemail message threatening the paper with legal action and expressing outrage about its plans to publish the story.
According to Bild, Mr Wulff, who was on a tour of Gulf states at the time, rang Mr Diekmann again two days later and apologised for the tone and content of his voice message.
A spokeswoman for Mr Wulff, quoted by German ARD television on Tuesday, said the president regarded media freedom as "a valuable commodity". She said Mr Wulff would not comment on private phone calls.
Bild says Mr Wulff had also called Mathias Doepfner, president of Bild's publishing house Axel Springer AG, and had urged him to put pressure on Bild to suppress the article. But Mr Doepfner had refused to do so.
The businessman whose wife made the loan to Mr Wulff was entrepreneur Egon Geerkens.
An interest rate of 4% was agreed - one percentage point lower than the usual bank rate, Bild said.
In February 2010, at around the time he faced questions over his ties to the Geerkens, Mr Wulff replaced the private loan with a bank mortgage.
On 22 December he issued an apology, saying he should have disclosed the private loan before taking office as president.
An editorial in Financial Times Deutschland called for his resignation on Tuesday, saying he had failed to act appropriately for the office of president.
"Everything taken together, it's slowly becoming too much... his credibility has been damaged by this plethora of mistakes. A man who is dragging around such a burden can no longer remain president," it said.
The influential daily Sueddeutsche Zeitung also said he should quit.
"The naivety and brazenness of Wulff's actions are worrying. He's not chief administrator of Osnabrueck, nor is he prime minister of Lower Saxony any longer - he is head of state. This post is apparently too big for Wulff," the paper said in an editorial.