(Updates with details, analysts comment, background)
BERLIN, March 1 (Reuters) - German Defence Minister Karl-Theodor zu Guttenberg will resign on Tuesday, a government source told Reuters.
Earlier, Bild newspaper reported that Guttenberg would step down. A spokesman for the minister did not comment on the report. He said Guttenberg would give a statement at 11:15 a.m. (1015 GMT).
Guttenberg, the most popular member of Chancellor Angela Merkel's cabinet, has been embroiled in a plagiarism row. Merkel had defended Guttenberg and analysts said he was important for conservative party hopes in three state elections later this month.
For an analysis on Merkel and Guttenberg: [nLDE71R1HDT]
He was stripped of his doctorate after admitting last week his PhD dissertation was flawed. He apologised to parliament after being accused of copying parts of the dissertation without correct attribution.
He had rejected charges of plagiarism and opposition calls to resign over the affair, which has dominated headlines and television chat shows for the last week.
Merkel had backed Guttenberg. Opposition leaders said she did not fire him over fears it would anger conservative voters, especially in the southwestern state of Baden-Wuerttemberg.
"It's bad for Merkel and she stuck with him far too long," said Konrad Jarausch, a political scientist at Berlin's Free University. "It's a major about-face for her. There will be some damage to her too over this."
Guttenberg, 39, has long been the most popular minister in Merkel's cabinet. But the aristocrat's popularity was based on his carefully nurtured image for honesty and integrity. German media have found scores of copied passages in his dissertation.
He said he made mistakes but not deliberately.
In recent days some conservatives distanced themselves. Education minister Annette Schavan on Monday called Guttenberg's actions shameful and parliamentary president Norbert Lammert said it was "a nail in the coffin for confidence in democracy".