Polls have opened in Germany and Chancellor Angela Merkel is expected to win a third term at the helm of Europe's biggest economy.

Germans are to elect their representatives to the Parliament or Bundestag, which in turn chooses the chancellor.

Opinion polls put Merkel's Christian Democratic Union at about 40%, while her centre-left Social Democratic Party (SPD) challenger, Peer Steinbrück is expected to win 27% of the vote.

However, support for Merkel's current government coalition partner, the pro-business Free Democratic Party (FDP), has slumped from nearly 15% in the 2009 election to 5.5%, barely clearing the 5% hurdle needed to get into parliament.

Merkel might therefore be forced to change partner and strike a coalition with the SPD, in a repeat of her first government in 2005, in which Steinbrück served as finance minister.

"I ask the people in Germany for a strong mandate so that I can serve Germany for another four years, make policies for ... a strong Germany, for a country that is respected in Europe, that works for Europe; a country that stands up for its interests in the world but is a friend of many nations," Merkel told her last rally before the vote.

A leftist coalition is also is also a possibility. The SPD, the Greens and the far-left Left Party together are polling at 44.5%, a figure that puts them in a dead heat with an SPD-FDP coalition.

However the SPD and the Greens have both so far ruled out striking any deals because the Left Party is seen as an uncomfortable ally due to its hard-line policies - the party evolved from the communist party of the former East Germany.

"It's not a game," Steinbrück said. "Don't believe it's decided yet - it isn't. I would ask for the voters' decision to be respected, because it's them, not political polls or certain observers, who decide an election."

Nearly 62 million people were eligible to vote. Polls opened at 08:00 local time (06:00 GMT) and results are due hours after closing time at 16:00.

No single party has won a majority in Germany in more than 50 years, and elections usually produce weeks of coalition talks before a government is agreed on.

A voter casts his ballot in BerlinReuters
Voters wearing traditional Bavarian costume at a polling station in Gaissach near Bad Toelz, southern GermanyReuters
German Chancellor and conservative Christian Democratic Union (CDU) leader Angela Merkel is the election favouriteReuters
Two nuns cast their vote in Germany federal election 2013Reuters
Social Democratic Party (SPD) candidate Peer Steinbrück casts his ballot in Bonn accompanied by his wife GertrudReuters
Voters wearing traditional Bavarian costume cast their ballots in GermanyReuters
The Reichstag, the seat of the German lower house of parliament Bundestag in BerlinReuters