Germany's defence minister Karl-Theodor zu Guttenberg has resigned after being unmasked as a plagiarist. The 39-year-old aristocrat, a baron who was widely tipped as a future chancellor, handed in his notice to Angela Merkel following almost two weeks of front page stories about the authenticity of his PhD thesis. "It is the most painful step I've ever had to take," Guttenberg said in a phone interview on the news channel N-TV. Last week, the University of Bayreuth stripped Guttenberg of his doctorate after he admitted (albeit inadvertently, he said) copying substantially from other sources. He said the errors arose because he was so busy.