Germany's defence minister has quit after he was accused of plagiarising parts of his doctoral thesis.

Karl-Theodor zu Guttenberg, 39, said he had decided to go "not just because of my faulty doctoral work" but because the persistent focus on it threatened to overshadow duties such as overseeing a major overhaul of the German military and troops' deployment in Afghanistan.

"It is the most painful step of my life," he said.

"Because my office, the Bundeswehr, academia and the parties that support me faced potential damage, I am drawing the consequences that I have and would have demanded of others."

Bayreuth University revoked Mr Guttenberg's academic title last week, saying the minister had "seriously violated" its standards by failing to credit sufficiently some of his sources, but the minister at first sought to cling on.

Mr Guttenberg has been the rising star of Germany's centre-right over the past two years. He built a reputation as a plain-speaking man of action in a brief stint as economy minister and then, after Germany's 2009 election, as defence minister.

In that job, he pushed through a plan to end conscription - part of an effort to slim down the German military and make it better adapted to an era in which it faces growing demands to deploy overseas.

But his crisis management after the plagiarism allegations emerged two weeks ago, and then kept coming, was less impressive.

He initially issued a statement describing them as "absurd," then said he would stop using his title as a doctor only temporarily while Bayreuth University looked into the accusations.

Chancellor Angela Merkel stood by him, saying a week ago: "I appointed Karl-Theodor zu Guttenberg as defence minister, I wasn't appointing an academic assistant."

But the scandal would not go away, raising the possibility that Mr Guttenberg would be a liability rather than an asset in upcoming state elections.

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