German President Christian Wulff has announced his resignation, after prosecutors called for his immunity to be lifted. An ally of Chancellor Angela Merkel, Mr Wulff, 52, stepped down over corruption claims involving a dubious home loan. German media say the crisis is unprecedented in post-war Germany. The president's role is largely ceremonial, to serve as a moral authority for the nation. The developments of the past few days and weeks have shown that [the German people's] trust and thus my effectiveness have been seriously damaged," Mr Wulff said in a brief statement. "For this reason it is no longer possible for me to exercise the office of president at home and abroad as required." At the centre of the row is the story - first published by the Bild newspaper - that Mr Wulff received a low interest 500,000 euro loan (417,000; $649,000) from the wife of a wealthy businessman in October 2008. Mr Wulff, who previously was premier of Lower Saxony, was later asked in the state's parliament if he had had business relations with the businessman, Egon Geerkens, and said he had not, making no mention of his dealings with Mr Geerkens's wife. The president was also heavily criticised for trying to force Bild not to break the story in the first place. It has emerged that he left an angry message on Bild chief editor Kai Diekmann's phone, saying the story must not be published. There were also corruption allegations against Mr Wulff, involving receiving political favours and free holidays from business executives.