Embattled German Chancellor Angela Merkel suffered a blow to her authority yesterday as her pick for president needed three voting rounds to win election after a rebellion in her ruling coalition.

Christian Wulff, 51, a conservative Merkel ally who once described himself as no "alpha male," secured 625 votes in a special electoral assembly, enough to become Germany's youngest-ever president.

His election was greeted by huge cheers from Merkel supporters after the longest presidential vote in German history, stretching into around nine hours.

But in what business daily Handelsblatt dubbed a "debacle" and a "first vote of no-confidence" in the weakened Mrs Merkel, Mr Wulff failed to win enough votes in the first two voting rounds despite her coalition's majority.

Bild, the mass-circulation daily, which had billed the election as a "vote of destiny" for the weakened chancellor and her unpopular government, called the result a "sensation" and a "massive slap for the ruling coalition."

Influential weekly Die Zeit called the result a "humiliation" for Mrs Merkel's coalition while Tagesspiegel daily described the vote as a "putsch" against the chancellor during what it termed the "day of the long knives."

Spiegel magazine said on its website it was "Merkel's biggest failure."

After the first two embarrassing voting rounds, the third became a battle for the political future of Mrs Merkel, four times named the world's most powerful woman by Forbes Magazine.

A recent poll in Bild showed 48 per cent of Germans wanted her to throw in the towel if her man had lost the vote, compared to 30 per cent who believed she should soldier on.