FRANKFURT (Reuters) - Czech investment group PPF has raised its stake in German media group ProSiebenSat.1 to 13.1% in stock and financial instruments, a regulatory filing showed on Monday, less than two weeks after a selloff in the German media company's shares.

ProSieben's shares plunged in late April after the company said prosecutors were checking the accounts of its voucher subsidiary and it would cut its dividend for 2023.

PPF, ProSieben's second largest shareholder, first disclosed a 9.1% stake in ProSieben in February, which it increased to approximately 12% in late April. It has also requested a seat on the German company's supervisory board.

Italy's MediaForEurope, ProSieben's top shareholder, has expressed concern about developments at ProSieben whose new management has announced a change in strategy and job cuts.

In addition, ProSieben's profit margins have come under pressure from falling advertising sales over recent months.

Under German law, if an investor's stake exceeds 30%, it has to launch a bid for the whole company.

(Reporting by Christoph Steitz; Editing by Susan Fenton and Barbara Lewis)