BERLIN- Germany's defense minister on Monday gave up his academic title following allegations that he plagiarized a sizable part of his doctoral thesis.

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Karl-Theodor zu Guttenberg acknowledged during a speech in Kelkheim near Frankfurt that he has made "grave mistakes."

The decision to renounce the academic title is the latest twist in a scandal that has become an embarrassment for Angela Merkel's conservative government.

The 39-year-old Guttenberg is Germany's most popular politician, according to opinion polls, but the plagiarism flap has prompted calls for his resignation.

When the accusations that Guttenberg copied tracts of his 475-page thesis from other sources without credit first emerged last week he decided to "temporarily" refrain from using the title pending the outcome of a review of the thesis by Bayreuth University of his thesis.

Guttenberg has previously admitted "mistakes" but strongly denied the allegations of plagiarism.

He finished the doctoral thesis in 2007, five years after becoming a lawmaker for Merkel's conservative bloc. The thesis _ "Constitution and Constitutional Treaty: Constitutional Developments in the U.S. and EU" _ was accepted by Bayreuth University, which is now looking into the matter.

He said it was "embarrassing" that he omitted to credit the newspaper article he used for the first paragraphs of his thesis' introduction, German news agency DAPD reported.

"On some instances, I possibly lost control of the sources," Guttenberg told party members.

German news magazine Der Spiegel said that Guttenberg had copied tracts from other sources like newspaper articles or research papers without credit on at least 62 instances. Newspaper Sueddeutsche Zeitung said his parliamentary research assistants had helped with some 20 more pages.

Merkel has backed her minister, saying on Monday she didn't hire him as an academic but as a defense minister.

Guttenberg rejected calls to step down. "I will do my job with all my strength," he told the gathering.

While the scandal is seen as an embarrassment for the government, Guttenberg still appears to have people's support. A weekend poll for ARD television found that 74 percent of respondents thought he shouldn't quit and only 22 percent said he should go.