BERLIN - Germany's defense minister, a rising star in Chancellor Angela Merkel's conservative bloc and the country's most popular politician, resigned Tuesday over allegations he plagiarized his doctoral thesis.

German Chancellor Angela Merkel holds a telephone receiver at the Siemens stand during the opening at the CeBIT computer fair in Hanover March 1, 2011. [Photo/Asianewsphoto]

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Karl-Theodor zu Guttenberg told reporters it was the "most painful step of my life," but he felt that he could no longer carry out his duties as minister amid the intense debate surrounding the work and his person.

Guttenberg admitted last months making "grave mistakes" in his 2007 thesis and gave up his academic title of "Dr." but insisted he did not intentionally copy the work.

Following that, Bayreuth University formally removed his academic title in order to uphold the institution's standards, which he said Guttenberg had "seriously violated" by failing to sufficiently credit the source of his work.

But the pressure continued to grow, with the scandal dominating the country's news coverage, despite polls showing the majority of Germans backed him.

On Monday, some 23,000 German academics joined the rising chorus of people demanding his resignation, sending Merkel a joint letter protesting her decision to keep Guttenberg on.

"I was always ready to fight, but have to admit I have reached the limit," Guttenberg told reporters at the Defense Ministry.