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German Defence Minister Karl-Theodor zu Guttenberg has temporarily renounced his doctorate title following accusations of plagiarism.

German media allege several passages of his 2006 thesis were lifted from other people's work.

The minister strongly denied the claims. He said there were undoubtedly mistakes and he was unhappy about them.

Chancellor Angela Merkel had offered to support him if he explained his actions and apologised, reports say.

But her chief spokesman Steffen Seibert made no comment on the defence minister's statement at a Berlin news conference, prompting journalists to walk out.

"I will temporarily, I repeat temporarily, give up my doctoral title," Mr zu Guttenberg told reporters in his statement minutes earlier.

"My dissertation is not a work of plagiarism and I completely reject all such accusations," he said.

He said he would "actively" help his former university as it investigated the claims and insisted he did not want to be treated according to different standards from anybody else.

Mr zu Guttenberg has come under intense pressure because of the claims from the media and political opponents.

The charismatic 39-year-old minister with an aristocratic background, seen as a possible successor to Angela Merkel as chancellor, had managed to avoid the public glare on Thursday because of an unannounced visit to northern Afghanistan.

But on his return, he cancelled a trip to an election rally in Saxony-Anhalt, shortly before he was due to give a speech.

Instead, he was called to an evening meeting with Mrs Merkel in Berlin to discuss the swirl of allegations which has led to the ZDF television website dubbing him "Zu Copyberg", Financial Times Deutschland naming him "Baron Cut-and-Paste", and Berlin daily Tageszeitung nicknaming him "Zu Googleberg".

The affair erupted when a law professor from Bremen University began writing a review of the thesis, with the aid of the internet.

The popular minister had rejected as "absurd" initial reports in the Suddeutsche Zeitung that one passage was copied word for word from a newspaper article and another was taken from a public lecture, without attribution.

Spiegel magazine reported that the minister had also used a paragraph from the US Embassy website without attribution and the list of alleged instances of plagiarism has continued to grow as journalists and internet-users pore over its contents.

The University of Bayreuth, where he completed his PhD on constitutional developments in the US and EU, has given him a fortnight to respond to the allegations in writing.

After calling an impromptu news conference on Friday, the beleaguered minister accepted that there were errors in his work but that did not amount to plagiarism.

"It was compiled over the course of about seven years while I was at the same time working as a member of parliament and while I was a young father. It contains mistakes, no doubt about it. I am extremely unhappy about every single one of those mistakes."

The opposition Social Democrats have warned that if the allegations of plagiarism are found to be true then the defence minister will have to stand down because of the damage to his credibility.