Porsche SE fell the most in six months in Frankfurt trading after the sports-car maker's family owners agreed to pursue a merger with the Volkswagen AG unit, ending a bid to boost its stake in Europe's largest automaker by acquiring shares.

Porsche slumped as much as 9.82 euros, or 17 percent, to 47.13 euros, the steepest intraday decline since Oct 27.

As part of the pact, the Porsche and Piech families, which together control half of Stuttgart, Germany-based Porsche, will take part in a stock sale that may raise as much as 5 billion euros ($6.7 billion), a person familiar with the plan said yesterday. Volkswagen gained 1.5 percent.