By Annika Breidthardt
BERLIN (Reuters) - Germany's education minister resigned on Saturday after being stripped of her doctorate for plagiarism, embarrassing Chancellor Angela Merkel and depriving her of a close ally in the run-up to a September election.
Annette Schavan quit four days after the University of Duesseldorf ruled she had "systematically and intentionally" copied parts of her thesis, and withdrew the Ph.D it had granted her more than 30 years ago.
It was the second time in two years that Merkel had lost a cabinet minister in a scandal over academic cheating. Karl-Theodor zu Guttenberg resigned as defense minister in 2011 after being exposed for plagiarizing his thesis - behavior that Schavan condemned at the time as 'shameful'.
The chancellor, who rarely displays her emotions, turned to offer Schavan a consoling smile at a joint news conference where she told reporters: "I accepted this resignation with a very heavy heart."
Schavan's departure is unlikely to weaken Merkel's chances to win a third term in elections on September 22. Her Christian Democrats regularly poll above 40 percent, giving them an easy lead over the main opposition Social Democrats.
But losing her key confidante is still a blow to the German leader, and forces her to make her fifth cabinet change of the current four-year term by bringing in Johanna Wanka, a former education minister in two German states, to replace Schavan.
Andrea Nahles, general secretary of the main opposition Social Democrats, said the government was "at its end".
Juergen Trittin, parliamentary leader of the opposition Greens, said Merkel had made a "glaring false start" to the year, with defeat in an important regional election and a budget row over a planned new train station in Stuttgart.
As Schavan was education minister, the charges were a direct threat to her credibility and, had she not resigned, would have threatened to overshadow Merkel's election campaign.
She said their friendship would last and reiterated she would fight the university's annulment of her doctorate, a distinction which carries prestige in German business and politics as well as in academic circles. Losing it means Schavan is left without any academic title as her degree led directly to a doctorate, not a bachelor or master's.
"I will not accept the decision and will take legal action against it," Schavan told reporters. "When an (education) minister sues a university, then that comes with strains, for my office, for the ministry, the government and for the Christian Democrats. I want to avoid just that."
Since Guttenberg stepped down two years ago, other German politicians have also lost their academic titles as a group of anonymous Internet activists took to examining doctoral dissertations on websites in search of plagiarism.
Merkel has survived a range of high-profile resignations without sustaining lasting damage. Early last year, disgraced president Christian Wulff, whom she had hand-picked for the job, stepped down over his financial ties with a wealthy businessman.
(Reporting by Berlin bureau, writing by Annika Breidthardt; Editing by Mark Trevelyan)