By Gareth Jones

BERLIN (Reuters) - Angela Merkel's education minister said on Wednesday she would take legal action against a decision to void her doctorate for alleged plagiarism, an untimely distraction for the German chancellor ahead of September's national election.

German opposition lawmakers said Annette Schavan, a close Merkel ally, should resign after the University of Duesseldorf said on Tuesday that parts of her 1980 doctoral thesis had been copied and that it was stripping her of her PhD.

Her case closely mirrors that of Karl-Theodor zu Guttenberg, who quit as Germany's defense minister in 2011 over a plagiarized thesis. Guttenberg had been viewed before his departure as a possible heir to Merkel.

"I will not accept the decision of the University of Duesseldorf and I will file a lawsuit against it," Schavan, 57, told reporters during a visit to Johannesburg, South Africa.

She declined to make any further comment for legal reasons.

The accusations of plagiarism are especially embarrassing for Schavan because she oversees Germany's universities and had previously been scathing in her criticism of Guttenberg, who resigned a month after losing his doctorate.

"An education minister who is proven to have grossly violated academic rules cannot continue in the post," said Renate Kuenast, a leading member of the opposition Greens.

"I assume that Frau Schavan will spare herself and education a prolongation of this affair by resigning."

Merkel's spokesman said the chancellor was in contact with Schavan and continued to offer her support.

"The chancellor values the minister's performance highly and has full trust in her," Steffen Seibert told a government news conference on Wednesday. Merkel and Schavan would have talks "in peace and quiet" after the minister's return from South Africa.

POLITICAL RIPPLES

Some members of Merkel's centre-right coalition said the minister had fallen victim to a politically motivated campaign to damage the government ahead of the autumn federal election.

Merkel, who holds a doctorate in physics, is Germany's most popular politician and her conservatives are tipped to win the September vote, but there is uncertainty about the make-up of the next government.

Merkel's current coalition partner, the liberal Free Democrats, may fail to clear the 5 percent threshold to enter parliament. That would force her to consider an unwieldy pact with the opposition Social Democrats.

German media were mostly critical of Schavan.

"If the education minister has cheated in her doctoral thesis, it is like the finance minister secretly hiding away his money in Switzerland or the traffic minister driving a car while drunk," said the top-selling Bild newspaper.

"There is no alternative (to resignation) for her."

The Duesseldorf university commission ruled that Schavan had "systematically and intentionally presented intellectual performance that in reality she did not generate herself".

The decision left Schavan without an academic title, an important symbol of status in German politics and business, as her degree program in philosophy finished solely with a PhD.

Since the allegations first arose in May last year, Schavan has repeatedly denied wrongdoing and said she wrote her dissertation with a clear conscience.

Her lawyers have said the proceedings of the commission had been riddled with mistakes and were unlawful, not least because information was leaked to the public in the process.

(Additional reporting by Shafiek Tassiem in Johannesburg and Madeline Chambers in Berlin; writing by Gareth Jones; editing by Mark Heinrich)