BERLIN, March 26 (Reuters) - Porsche SE, the holding company that controls Volkswagen, said on Wednesday it suffered a blow to its 2025 after-tax profit, weighed by costs at the German auto group and luxury sports-car maker Porsche AG.

Porsche SE reported adjusted earnings after tax of 2.9 billion euros ($3.35 billion), down by around 9% year on year.

The group's net debt fell slightly to 5.1 billion euros.

The holding company of Germany's Porsche-Piech auto dynasty is Volkswagen's largest investor with 31.9% of shares and 53.3% of voting rights. It also owns 12.5% of Volkswagen subsidiary Porsche AG. The top shareholder's profit was hit by billions of euros in costs after Porsche AG slammed the brakes on its electric vehicle rollout in response to weak demand, including in China where sales have slumped.

Despite the drop in adjusted earnings, Porsche SE said its smaller investments generated 193 million euros in profit last year, highlighting efforts to diversify its portfolio.

"Our unique network has become a key strategic asset that significantly contributes to the strong financial performance of our portfolio," board chairman Hans Dieter Poetsch said in a statement. With the German automotive sector under pressure from steep tariffs, Chinese competition and a costly shift to EVs, Porsche SE has said it is exploring defence investments to diversify its portfolio.

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