Germany's dull election race has been spiced up by Angela Merkel's main challenger, Peer Steinbrück, with a minor controversy over his appearance on the front cover of a news magazine in which he gives the middle finger.

Steinbrück, the candidate for chancellor for the largest opposition party, the Social Democratic Party (SPD), was pictured in the provocative pose on the front of Süddeutsche Zeitung magazine.

The snap was part of the magaizine's "Mute Interview" feature called Don't Say Anything Now, in which interviewees answer questions with gestures instead of words.

Steinbrück, who has a reputation for blunders, was asked about his many nicknames.

"Gaffe Peer, Problem Peer, Peerlusconi - you don't have to worry about being given any nice nicknames, do you?" said the interviewer to which Steinbrück responded with the finger.

His spokesman, Rolf Kleine, said the politician had been "a little too spontaneous" in his gesture. The publishers said the spokesman had tried to block distribution of the magazine but it was Steinbrück himself who eventually gave the green light.

"Words are not always necessary to be outspoken, for example when one is constantly questioned about dull issues instead of being asked about truly important matters," Steinbrück tweeted to explain his pose.

The photo was seized on by his opponents as evidence that he was not fit to run the country.

"That's unacceptable. You can't do that as a chancellor candidate," said Germany's economics minister, Philipp Rösler of the Free Democratic Party.

Germans vote on 22 September but the electoral campaign has failed to spark interest and has been described as the dullest in decades.

The SPD and Angela Merkel's Christian Democratic Union (CDU) both lack strong, distinctive policies. Merkel is almost sure of being re-elected for a third term.

"Germany is doing well today but we can't take this for granted," one of Merkel's campaign slogans reads. "I want us to be successful in the future," she says in another.