German Defence Minister Karl-Theodor zu Guttenberg resigned on Tuesday, after he became embroiled in a plagiarism scandal over his PhD, dpa reported.
"I informed the chancellor (Angela Merkel) in a very friendly conversation that I will withdraw from my political offices and asked for a discharge," Guttenberg said.
"It is the most painful step in my life," he added.
The minister had come under increasing political pressure following revelations that more than 100 pages of his 475-page doctoral thesis had been copied from other works without citation.
A major cabinet reshuffle is unlikely, as a successor to Guttenberg is likely to be sought in the ranks of his Bavaria-only Christian Social Union or within Merkel's Christian Democrats, their national allies.
Guttenberg said the media focus on him and his doctorate had diverted attention away from his role as defence minister, while German soldiers were fighting and dying in Afghanistan.
"If, at the cost of the soldiers, the debate is only about my person, I cannot take responsibility for this," Guttenberg said.
The office of defence minister required undivided concentration and faultless work, he added.
"I was always prepared to fight but I reached the limit of my strength," Guttenberg told journalists at the defence ministry.
Merkel had earlier in the day unexpectedly interrupted her visit to the opening of the CeBIT computer trade fair in Hannover to conduct a 10-minute telephone conversation, apparently about the resignation.
President Christian Wulff is said to have been informed of the decision.
The plagiarism allegations had first surfaced almost two weeks ago. Last week, the minister was stripped of his doctoral title - at his own behest - but he had so far remained popular and enjoyed Merkel's backing.
Bayreuth University , which gave Guttenberg top marks for his dissertation, is investigating whether he deliberately tried to mislead examiners by passing off others' work as his own - a charge the minister has denied.
The 39-year-old minister said it was a difficult decision to resign from the office into which he had put "his whole heart."
However he stressed that his decision was not purely down to his "faulty doctoral thesis," but rather due to the media storm this had generated, which prevented him from meeting his responsibilities as defence minister.
He added that he held back his resignation in order to participate in a memorial service for three soldiers who had died in Afghanistan, and to finalize elements of a comprehensive military reform programme.
"The reform plan stands," he said, placing faith in his successor to carry it through.
He said the last weeks had not been easy on him personally.
"The enormous weight of the media attention to my person was not without effect on me and my family," Guttenberg said, but stressed that he did not expect pity, as he had chosen to enter politics.
The minister thanked Merkel, as well as all citizens, politicians and soldiers who had supported him during the past weeks.
Guttenberg has frequently been cited as Germany's most popular minister - ahead of Merkel - and was seen as a rising star in her government.
But over the past few days, support had begun to crumble even within Merkel's Christian Democratic Union, the sister party to Guttenberg's Bavaria-only Christian Social Union.