Chancellor Angela Merkel's conservatives suffered a crushing defeat on Sunday in an election in Germany's most populous state, exit polls showed, a result which could embolden the left opposition to step up attacks on her European austerity policies.

The election in North Rhine-Westphalia, a western German state with a bigger population than the Netherlands and an economy the size of Turkey, was held 18 months before a national election in which Merkel is expected to fight for a third term.

She remains popular at home for her steady handling of the euro zone debt crisis, but the sheer scale of her party's defeat respresents a heavy blow that could tilt the German political landscape and leave her more vulnerable to domestic critics.

According to an exit poll for public broadcaster ARD, the centre-left Social Democrats (SPD) won 39 percent of the vote and will have enough to form a stable majority with the Greens, who scored 12 percent.

The two left-leaning parties had run a fragile minority government for the past two years under popular SPD leader Hannelore Kraft, whose decisive victory on Sunday could propel her to national prominence.

Merkel's Christian Democrats (CDU) saw their support plunge to just 26 percent, down from nearly 35 percent in 2010, and the worst result in the state since World War Two.

The Free Democrats (FDP), a pro-business party that rules in coalition with Merkel's conservatives at the federal level, looked to have made it back into the state assembly, in what many will see as a rebound for the party after a collapse in support in recent years.

The upstart Pirates, a new party that campaigns for internet freedom, continued their strong run at regional level, making it into their fourth straight state parliament, the exit polls showed.