Karl-Theodor zu Guttenberg admits 'grave mistakes' in his dissertation but denies plagiarism allegations [Reuters]

Karl-Theodor zu Guttenberg, Germany's defence minister, has resigned over allegations he plagiarised his doctoral thesis, saying he could no longer carry out his duties as minister.

The popular politician, who was seen as a rising star in government, said on Tuesday he had "reached the limits of his strength" as he announced his resignation.

"I informed the chancellor in a very friendly conversation that I'm resigning from political offices and requested to be
relieved. It's the most painful step of my life," he said.

The 39-year-old had his doctorate title revoked after admitting last week there were "grave mistakes" in his 2007 thesis. He said he made some errors, but not deliberately, and insisted it was not plagiarised.

German media are said to have found scores of copied passages in his dissertation.

Guttenberg said he was resigning not just because of his "error-filled doctorate" but whether he could "still live up to the highest expectations" he placed on himself.

Angela Merkel, the German chancellor, had defended Guttenberg, and analysts had said he was important for their conservative party's hopes in the next elections.

But some conservatives distanced themselves from the politician in recent days.

Earlier this week Annette Schavan, the education minister, called Guttenberg's actions shameful, while parliamentary president Norbert Lammert described the plagiarism scandal as "a nail in the coffin for confidence in democracy".

Analysts have said the scandal is an "embarrassment for Merkel", who also lost another ally, central bank president Axel Weber, several weeks ago.

"It makes her look bad because she should have known he wouldn't survive this. She backed him even though she knew how fragile it all was," Gero Neugebauer, a political scientist at Berlin's Free University told the Reuters news agency.

The chancellor had been keen to avoid any turbulence ahead of three state elections and two summit meetings in Brussels this month.