Karl-Theodor zu Guttenberg on Wednesday dismissed as "absurd" the allegations reported by German daily Sueddeutsche Zeitung, insisting that none of his employees helped him draft the thesis. The minister said, however, he is willing to check whether there were any omissions or errors among the 1,200 footnotes in the 475-page thesis.
He finished the doctoral thesis in 2007, five years after becoming a lawmaker for Chancellor Angela Merkel's conservative bloc. The thesis - "Constitution and Constitutional Treaty: Constitutional Developments in the U.S. and EU" - was accepted by Bayreuth University.
The newspaper highlighted different tracts - adding up to several pages - from Guttenberg's thesis that it said were copied from other sources such as newspaper articles or presented without credit.
"The production of this thesis was my own achievement," Guttenberg insisted.
For months Guttenberg has been leading the polls as Germany's most popular politician, but has recently faced a series of scandals within the military.
Germany's Frankfurter Allgemeine Zeitung newspaper also said Guttenberg copied from one of its articles without credit. The paper claimed the first paragraph in Guttenberg's introduction was "almost word by word" the same as one of its articles published ten years earlier.
"It's just plain stupid to copy something like that," political scientist Barbara Zehnpfennig, the original article's author, told newspaper Handelsblatt.
A spokesman for Bayreuth University said it was looking into the allegations, the German news agency DAPD reported. Guttenberg could eventually lose his doctoral degree if an investigation concluded that the alleged omissions amounted to fraud.
Guttenberg's thesis supervisor defended the minister.
"The accusation is absurd, the thesis is not a plagiarism," Peter Haeberle told the German tabloid Bild, adding that Guttenberg was one of his best students.
But Sueddeutsche Zeitung also reported that Guttenberg had copied several paragraphs from a 2003 article in Switzerland's Neue Zuercher Zeitung am Sonntag. The article's author, Klara Obermueller, on Wednesday said the minister should admit his mistake.
"Guttenberg took a long passage from my article, so I don't believe it's a mishap," she told the German magazine Bunte.
The Swiss newspaper's editor-in-chief Felix E. Mueller asked Guttenberg to apologize for what he said was copying of more than 80 lines from Obermueller's article.
Mueller said that the text was "absolutely identical, with the exception of two words," but he wouldn't pursue the matter even if the minister did not acknowledge the fault.
"It's mainly Mr. Guttenberg's problem, not mine," Mueller said.
Frank Jordans in Geneva contributed to this report.