The Karlsruhe court ruled on Wednesday that the reforms pushed through by Merkel's coalition in November 2011 ran contrary to principles of equality and direct suffrage.

It also ruled that the changes did not offer an equal opportunity to all parties in a fair manner.

The result is viewed as a victory for the parliamentary groups of the opposition SPD and the Greens, as well as the more than 3,000 citizens who brought the case.

Under the current system, voters have a first vote, directly for a candidate, as well as a second vote that determines the distribution of seats in the German parliament.

There are exceptions, however, that allow so-called "overhang-mandates" - extra seats if a party wins more direct mandates than the number of seats it should get according to the percentage of votes.

The new rules were developed after a previous judgment by the Constitutional Court in July 2008. The court decided then that this current system could give an unfair advantage to dominant parties.

Angela Merkel's CDU regularly acquires the most direct mandates, and has benefitted from the "overhang mandate" regulation the most.

Her coalition had been accused of dragging its feet over seeking a consensus on the correction of the electoral law. The current reform was then pushed through parliament with the votes of the ruling parties alone.

Now they must all go back to the drawing board quickly. A new law must be decided before the country's next general election, in autumn 2013.

rc/rg ( Reuters, dpa, AFP)