Germany's top court on Wednesday ruled voting law reforms introduced by Chancellor Angela Merkel were unconstitutional, calling for changes ahead of elections next year.
The Federal Constitutional Court judged that reforms drawn up by Merkel's ruling coalition in 2011 "in several respects breach the fundamental principles of electoral equality and equal opportunities for political parties."
A new draft of the electoral law must now be drawn up before Europe's top economy heads to the polls, likely in September or October next year.
The centre-left opposition Social Democrats said the ruling was a "resounding slap for the government" while Merkel's deputy spokesman told a regular briefing that Berlin "took note" of the verdict.
The changes related to a complex area of German electoral law that can hand parties extra seats in the Bundestag lower house of parliament.
In the 2009 election, Merkel's conservatives won 24 extra MPs in the Bundestag by means of these so-called "overhang seats." The court said the number of such seats should be limited to around 15.
The court has frequently proven a thorn in Merkel's side and is preparing a hotly awaited ruling on key tools to fight the eurozone crisis on September 12.
German President Joachim Gauck has held off on signing into law a European treaty creating a permanent 500-billion-euro ($607-billion) bailout fund for crisis-hit countries until the court rules on a raft of legal challenges to it.