Germany's president says an angry call to a newspaper editor regarding a story on a private loan he received was a serious mistake, but is making clear that he has no intention of stepping down.

Christian Wulff -- Chancellor Angela Merkel's candidate for the largely ceremonial presidency in 2010 -- has faced questions over a euro500,000 ($650,000) loan he received from a wealthy businessman's wife before he became president.

Wulff previously apologized for mistakes in handling that. But pressure deepened this week with revelations he called the editor of Bild newspaper last month to complain about its plans to publish the original story about the loan.

On Wednesday, Wulff told ARD and ZDF television the call was "a serious mistake I am sorry for." But asked whether he had considered quitting, he replied: "No."