German Education Minister Annette Schavan has resigned after a university stripped her of her doctorate for plagiarism.
Duesseldorf's Heinrich Heine University voted last Tuesday to remove her doctorate following a review.
Ms Schavan, a close ally of Chancellor Angela Merkel, insisted she would still fight the university's ruling.
In 2011, Defence Minister Karl-Theodor zu Guttenberg also quit after allegations he plagiarised his thesis.
Johanna Wanka, the culture and science minister of the state of Lower Saxony, has been appointed as Ms Schavan's successor.
Part of the education minister's brief is to oversee German universities.
Addressing Mrs Merkel with the words "Dear Angela,", Ms Schavan thanked the chancellor for her friendship.
"I think today is the right day to leave my ministerial post and to concentrate on my duties as a member of parliament," the 57-year-old said.
"I will not accept the university's decision and will take legal action. I have never copied nor plagiarised. The accusations hit me very hard."
Ms Schavan added that her main priority was to protect her department and the government.
"First the nation, then the party, then myself," Ms Schavan said, citing words used by former state prime minister Erwin Teufel.
"When an education minister sues a university, then that comes with strains, for my office, for the ministry, and for the Christian Democrats. I want to avoid just that."
The university of Dusseldorf decided to look into Ms Schavan's 1980 doctoral thesis after an anonymous blogger raised questions about it.
The faculty committee found she had "systematically and intentionally" copied parts of her thesis, Person and Conscience.
In a statement declaring the doctorate invalid and withdrawing it from Ms Schavan, the faculty head Bruno Bleckmann said they had "decided by secret ballot, by 12 votes to two, with one abstention".
Ms Schavan had been scathing in her criticism of Mr Guttenberg when the scandal of his plagiarism broke.
"As someone who was herself awarded a doctorate 31 years ago and who has supervised several doctoral candidates, I am ashamed and not just behind closed doors," she told the Sueddeutsche Zeitung at the time.
Analysts say her resignation will be hugely embarrassing to Mrs Merkel.
The chancellor is facing federal elections on 22 September.
She said she accepted the resignation "with a heavy heart".
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